What a Telescope Does
A telescope is just like a camera lens. Therefore, its most basic components are a focal length and an aperture. The larger the aperture, the more light it will collect, and the brighter the object viewed will be.
The longer the focal length, the smaller the field-of-view (FOV) will be. This can be changed by changing eyepieces where the smaller the eyepiece, the narrower the FOV. This means that any telescope can have any magnification or "power," but it'll be as dark as pitch if the aperture is too small. A rule of thumb is that you can get about 40x practical "power" per 1" of aperture.
What to Avoid
Department-store telescopes. They are over-priced and under-built.
Anything with an aperture smaller than 3" (7.5 cm). Especially when they advertise stuff like "400x magnification power!!!!"
What to Look For
Buy the largest aperture you can afford. 4" or larger.
A sturdy tripod and a sturdy mount between the telescope and the tripod.
While Dobsonian telescopes are cheaper than equatorial and alt/az (altitude/azimuth), it's the equatorial ones that will have a clock-drive to compensate for Earth's motion. This is necessary for astrophotography of anything other than the moon, sun, star trails, artificial satellites, or meteor showers. So you should look for an equatorial mount with a clock-drive.
If you can't find objects yourself, you'll want one of those Go-To things, but they're expensive so you're better-off learning to use RA/DEC to find objects yourself, or to starhop.
To Mount a Camera
You need both a T-mount and a T-ring. The T-mount is specific to the type of eyepiece the telescope takes, either 1.25" or 2" usually. It will be a tube, much like an eyepiece, but it will end in a threaded region.
The threads fit into one end of the T-ring which is specifically made to fit your camera body. One end of the T-ring is universal and screws into the T-mount, the other end goes on your camera body like a normal lens ... which is what the telescope technically is, just a really long lens with a relatively large aperture. Obviously any auto-focus will not work, and usually the auto-exposure won't work.
The Meade ETX-90 looks to be a tad slow at f/13.8, but that's expected given its aperture of 3.5". A focal length of 1250 mm will give you a field of view (with a 35 mm eyepiece) of about 1.6°, which is 3x the size of the full moon. Stick a 9 mm eyepiece in there (the very smallest I recommend or you'll go blind trying to stare through that tiny hole) and you have 0.4°, or 24 arcmin. Planets are usually measured on the scale of arcsec, which are 1/60 of an arcmin.
