Rooting doesn't give you extra battery. It's just there's things you can do once rooted to optimize your phones performance that you couldn't do before. Out of the box they're set pretty conservatively.
Just google "root HTC desire" or whatever your phone is, probably return you to this forum lol
There's a thing called Set vsel which let's you adjust clock & power settings, some custom roms have this built in. After you've rooted you can search for "set vsel settings < your phone >. You might find it here or another forum. You'd either need to download this app (requires root) or install a custom rom that has it (like cyanogenmod).
Read up on what most people are using to get a safe 'ball park' figure that won't harm your phone.