you need to know which software version your phone has.. and you need to know which hboot your phone has.
software version menu>settings>about phone>software info> software number
hboot..turn off phone.. press volume down and hold pwr button. at top find hboot version. (to get out just vol down to 'reboot' then press pwr)
this information will determine which root method you can use. this thread might be of help:
SprintUsers.com - View Single Post - EVO Custom ROM's Thread
various root guides can be found here:
EVO 4G Android Development - xda-developers