I've been using my Meade 90 ETX for a few years trying to become a better astronomer and am familiar enough with the basics to navigate my way around. I was wondering in what way an amateur astronomer such as myself can contribute to the scientific community. I would like to try and do more than just stargaze and take pictures.
Amateurs do useful scientific work by being many-handed and widely spread.
Many Supernovae are discovered by amateurs. You need a set-up that can image one galaxy after another and look for any "new stars" that appear in them. As you are looking for the appearance of a Mag-14 star you need a big mirror; 90mm might not be enough. However look to Backyard observatory supernova search for more information. With the right equipment, you might find 1 SN every 5000 galaxies imaged.
You can also contribute by observing asteroid occultations. These occur when an asteroid passes in front of a star, blocking out its light. By timing the occultation you can get an idea of the size of the asteroid in one cross-section. If multiple people observe the same occultation from different locations you can get a picture of the shape of the asteroid. See the Asteroid occultation site which links to FAQs and a page for submitting reports.
You can also help the study of variable stars and in particular their light curves - You do not require a big scope for this. See for example the American Association of Variable Star Observers, or the British Astronomical Association Variable Star Section.