Go over to Android forums. Navigate to the section for the Galaxy Prevail. Go to the All Things Root section.
There they have Odin images. Use the latest Odin image to install a recovery and a rooted stock rom on your phone. I think the latest one is the FE16 image. If you do not have the latest baseband (fe16) then it'll update your baseband too.
Once there you can go about doing stuff to your phone. If you're not comfortable doing it yourself then you can install CTMod from the recovery that Odin put on your phone. CTMod is the smallest fastest rom the Prevail has. It is VERY minimalistic. It has market but you'll have to install basic stuff to make it functional, like maps, gmail etc.
I'd suggest partitioning your SD card so you can use that part of the SD card for storage. My sd card is 16gb so I have a 2gb partition that is added to my phone's storage. Making my phone's "internal" memory 2gb+. You'll find partitioning stuff in the same section of the forum or you can do it through CWM recovery too. Your SD card will be wiped if you partition it. Save your stuff!
There's also information on how to have a 3rd partition so it can be used as swap space for your phone (like a page file for windows) but I've never tried that. I feel that all the writing back and forth is going to make my sd card die early... and with 16gb worth of space I stand to lose a lot.