The camera sensor isn't at focus and isn't able to come to focus due to the extra distance inside the camera to reach the image sensor. This is a fairly common issue when using a DSLR camera with a Newtonian reflector.
Here's why it happens:
The focal length of the telescope is 300mm. Light starts focusing when it hits the primary mirror at the back of the telescope. The also reflects the light forward toward the secondary mirror (mounted at the 45° angle to bounce light toward the eyepiece).
When an eyepiece is inserted into the barrel of the focus assembly, the focuser allows you to move the eyepiece closer or farther until the you reach the 300mm focal point.
When you use a camera, such as your Canon EOS, the camera itself has a 44mm "flange to focal plane" distance. The imaging sensor inside the camera is at the back of the camera -- behind the reflex mirror. Space is needed for that mirror. Add the thickness of the T-adapter (maybe that's another 5mm) and you're at about 49mm -- roughly 2".
If you adjust the telescope focus all the way "in" to bring the camera as close as possible ... you'll notice that it is getting better, but you run out of travel on the focuser before achieving focus.
With other telescopes ... such as a traditional refractor, the telescope has a 90° "diagonal" on the back of the telescope to let you look down into the eyepiece rather than having to get on the ground and try to look up (thus saving you from back pain). The distance that light travels through that 90° diagonal is considered part of the overall focal length of the scope and it adds something close to 2" (about 50mm ... give or take) to the overall focal length of the scope. THIS MEANS that you can remove the 90° diagonal. This subtracts ~50mm of focal length. Then attach the camera directly to the back of the scope (shooting "straight through" without a diagonal) and that adds ~50mm of focal length ... so you're basically back to the correct focal length and are well within the focus travel of the focuser to bring the image to sharp focus. The same is true of most scopes where the eyepiece is attached to the back via the 90° diagonal.
The problem with a Newtonian reflector (such as your Celestron First Scope) is that the focuser is at the front of the telescope on the side of the optical tube. No 90° diagonal is needed -- it's effectively built-in to the telescope.
General Info on Astrophotography Exposures
Before you plunge too deep or invest too much ... many objects in the night sky are not very bright. This means cameras need to use longer exposures to capture them. Imaging things like galaxies and nebulae often mean taking exposures that may be 5, 10, or even 15 minutes in some cases (or longer). The Earth is spinning and that means these objects would be moving through your field of view while the camera sensor is capturing an exposure ... resulting in a smeared image.
To compensate, telescopes need to rotate to counter-act Earth's spin (typically these telescopes are mounted on an equatorial tracking mount). This is not something you'd be able to do with your Celestron First Scope.
But all is not lost ... there are some objects bright enough that the exposures are very fast. The Moon and bright planets use very short exposures and can be captured using your scope.
When capturing planets, the technique is to capture a series of raw video frames (e.g. perhaps 30 seconds worth of video) and feed the data through planetary image stacking software. e.g. applications such as AutoStakkert are free (PC only ... not available on mac) and do this quite well.
Newtonian reflectors are very popular as visual telescopes, but less popular for astrophotography due to problems reaching focus. There are other optical designs with are more popular for astrophotography.
There is such a thing as a Newtonian Astrograph. These are Newtonian reflectors designed for astrophotography -- and these telescopes can achieve focus with a DSLR attached.
They do this primarily with one simple change ... the only component on a Newtonian reflector which focuses light is the primary mirror. They position the primary mirror about 2" closer to the secondary mirror. By shortening the tube, they've created an extra 2" after the focuser tube such that a camera would still achieve focus. To use an eyepiece with such a scope, the eyepiece needs to be positioned about 2" farther away ... so they generally provide a short 2" extension tube. You use the tube when using an eyepiece, and don't use the tube when using a camera.
One nuance of Newtonian astrographs is that since the primary mirror projects the focusing light forward in a cone-shape path, bringing the secondary mirror closer to the primary mirror means that some light will spill over the edges of the secondary mirror (it wont reflect all of the light). To compensate, the secondary mirror is usually a tiny bit larger. But of course this means you have a large central obstruction -- which also blocks a tiny bit more light.
That option requires getting a different type of Newtonian reflector to use with the camera. There are a other options.
Since the focuser travel is designed to allow you to position an eyepiece at correct focus, you can put a barlow lens in the focuser. A barlow is a focal length multiplier. They come in various versions such as a 2x barlow, or 3x barlow ... or 4x barlow. A 2x barlow is the most common.
You get some distance beyond the barlow for the eyepiece, but this is where you can attach the camera instead of an eyepiece and this often solves the problem -- with some side-effects.
One of the side-effects is that you've changed the effective focal length and focal ratio of the instrument. Objects will appear larger and dimmer (you've spread the light across a larger area) ... but recognize that you make everything bigger ... including the amount of blur. Magnified images usually wont appear quite as sharp. (There is no free lunch.)
There is a special variant of barlows that use tele-centric optics (specifically "image-space tele centric"). A normal barlow projects light back in a cone-shape. You only get the stated magnification at the correct distance. But a tele centric barlow projects light back as parallel rays ... such that the distance away from the barlow lens no longer matters. You'd get the same image whether the camera was 1" back ... or 10" back. These tend to be more expensive (several hundred dollars). A basic barlow isn't using particularly expensive -- some are less than $50 USD and most are less than $100 USD.
You can also get something called an Eyepiece Project Adapter. This is a larger diameter tube that is large enough to allow you to insert one of your existing eyepieces inside it. The back of the tube mounts to the camera body (using your T-adapter) and the eyepiece "project" the image onto the camera's focal plane.
Up to this point, the options let you use your existing camera. Here are some options that require a different camera.
The eyepieces for your telescope project light back in a cone-shape. If you use a smartphone, and hold the phone so that the camera lens in at the correct distance and in the center of that cone, the camera can usually focus and capture an image.
Doing this via hand-held use is a bit tricky and takes some practice. There are adapters that clamp around the eyepiece barrel and have a mounting bracket for your smartphone ... with adjustments that allow you to slowly move phone up/down, left/right, closer/farther ... until you find the magic spot. Celestron makes one that became popular called the Celestron NexYZ Universal 3-Axis Smartphone Adapter (about $75 USD).
There are some cameras where the sensor is nearly at the camera mounting flange ... they don't need an extra 2" of focus. They slip into the 1.25" (or 2") focuser barrel of your telescope and this makes things much easier.
They generally have tiny imaging chips ... and cost a fraction of what you'd pay for a DSLR camera. Cameras such as the ZWO ASI120MC-S or QHY QHY5L-II are both less than $150 USD (note that these have very tiny imaging chips ... suitable for planetary imaging.)
The ASI120MC-S has a flange-to-focal-plane distance of only 12.5mm (about 1/2"). The ASI120MC-S also includes a 150° all-sky lens. You can lay the camera face-up on a table outside and capture an image of the entire sky (everything more than 15° above the horizon). Some people use these to capture meteor showers.
The QHY5L-II is designed to look like an eyepiece and the image sensor actually slides inside the eyepiece barrel (there is no back-focus distance and they include a short extension tube just in case the camera sensor is too-close to achieve focus and it needs to be extended farther back.)
These cameras have no controls on them. They are powered by and controlled through their USB port. You must use a computer to control them ... you'll need a laptop, etc. to bring along when you go outside.