Not to be mean but if your asking that question you may need to do a lot more research on turbo's and the install of them on the Focus.
You control the amount of boost through a valve (Wastegate) which relieves the extra boost the turbo makes over what you want (can be mechanical or have a digital boost controller). Ideally you want a turbo for what psi you will be using (don't get a turbo that can do 40psi on the Zetec if your goal is 10).
2001 ZX3 2.0L Zetec-
Hatch Nation #42
Going through Rebuild See "The TICK Build Thread" for details (http://www.focusfanatics.com/forum/showthread.php?t=289168&referrerid=62897)