To start baking you need to stake a minimum of 14000 CCD (the new name for GTU). Currently, it is possible to buy CCD on switchere.com. There is also an airdrop happening where you can get some CCD for free on our telegram channel. However, CCD is not yet listed on exchanges, but will be soon.
Currently, it may not be worthwhile to become a baker with a relatively low stake. With the minimum 14k stake, you would expect to bake a block approximately 8 times per year right now. Currently the earnings from baking a block are around 268 CCD plus a share of the transaction fees. (Note that this is subject to change as CCD start to be distributed publicly and once listing takes place. Your earnings will depend on your own stake relative to the stake of all other bakers.) Our roadmap for the Sirius release has delegation, so you will be able to earn rewards without running a baker yourself.