There are different types of epoxy and the choice will depend on what are you trying to accomplish.

There is one part epoxy that is already pre-mixed and and is ready to be applied to a garage floor. And there is two part epoxy that comes in two separate containers and is mixed right before applying it to the surface. Two part epoxy is higher quality as it bonds better with concrete and it is more expensive than the one part.
If it is a to do it yourself project and you don't mind recoating your garage floor every couple of years, one part epoxy may work for you. And if you are hiring someone to do it, it only makes sense to choose a better quality epoxy.
One part epoxy cost about $30 per gallon and available at home depot. It can be custom tinted to just about any color. This epoxy will require a primer which is another $20.
Two part epoxy sold at home depot are Behr($72 per gallon) and Rust-Oleum($65 per gallon).They come only in two colors silver gray and tan. Rust-Oleum is better quality. It also does not require a primer. It comes in water or oil base and for higher traffic garage floors oil base can be used. Rust-Oleum will work well for most of the garage floors. For exterior application as driveway or carport higher quality epoxy will work better.
Most durable but at the same time most expensive epoxy is available at Arizona Polymer Flooring. It is about $200 for a gallon of primer and epoxy.
Preparation of the surface is the most important part of application process.
The best way to prepare concrete is to use Behr cleaner/etcher first,power wash the concrete after scrubbing it with Behr cleaner,use acid for etching afterwards and power wash concrete again. The next day after concrete is completely dry first coat of epoxy (or primer, depends on the product your are using) can be applied.
For showroom look a clear coat can be applied as a top coat a day after.