I have experience using this telescope. It looks like you can focus correctly if you get a good view of the moon. You probably don't want more magnification (the Andromeda galaxy is comparable in size to the moon). It is easier to get and keep targets in the field of view with a low power eyepiece, and I suspect that is your problem.
Start with planets. Use your low-power eyepiece, 20 mm. Jupiter is the easiest to see as a disk, but it is unfortunately close to the sun right now so it is low in the west in the evenings. With your 20mm eyepiece, it should be quite obvious as a disk with satellites and you may be able to see bands.
Mars is quite prominent right now in the evening sky but it is smaller than Jupiter. Still, with your 20mm eyepiece, you should be able to at least tell it is a disk (and a bright orange one) if you have it in the view of the eyepiece. Once you have it centered in view of your 20 mm eyepiece, you can try switching to your high-power (6mm) and see if that reveals any more detail. Markings on Mars or quite subtle; often it looks like a featureless orange ball.
If your night sky isn't that dark because you are close to a city with its outdoor lights, it can be a challenge to get a good view of even spectacular "deep sky" objects like the Andromeda galaxy. Again, use your low power eyepiece and no Barlow lens. The core of the galaxy is much brighter than the extended glow around it, so it can appear star-like. Try looking at with "averted vision" (don't look directly at it, to use the more sensitive light-detectors towards the edge of your field of vision) to pick out the faint glow that surrounds the core. If you go to a dark sky location (one where you can really see the Milky Way) the XT8 can give a pretty impressive view of the galaxy where the elliptical outer glow will fill the FOV of your low-power eyepiece.
If it is hard for you to travel to a dark-sky location, you can try buying a deep-sky filter. This filter tries to block the specific wavelengths thrown into the sky by outdoor lighting. It makes everything you see through the scope dimmer and kind of green, but it can improve contrast on galaxies and nebula by making the background sky darker.