Can be quite expensive to do as the cameras need special mounts which replace the lenses to stop light bleeding through and to ensure the entire subject is in focus. Depends what you want really as microscopic photos come in vastly different variations based on what you actually intend to look at. If you are simply talking about getting really close in to get better than macro quality, you should probably look at unocular microscopes which have a large observation area for a petri dish instead of a slide.
If you want to go down to cellular level, a standard light microscope which a white bulb and a camera mount would be required. You'd also then need either premade slides or your own kits to make your own slides. If you intend to start using stains, things can get pricey also.
I'd highly recommend having a google search for light microscopes and try find something in a comfortable price range which has a DSLR mount. Ask the shop selling them for info based on whichever the product you find is in relation to the quality of the photos and the standards used in the mount and you'll gain a far clearer picture of what you need.
