Since you Android Version is 2.3.5, you have HBOOT Version 2.18 and will have to use HTC Dev to unlock the bootloader. Carefully follow the directions at
http://forums.androidcentral.com/ht...26-guide-rooting-phone-hboot-2-18-0001-a.html. When you get to step 8, I recommend installing Amon RA available at
[Recovery] [09-Feb-2011] RA-supersonic-v2.3 - xda-developers instead of CWM Recovery. Follow all the remaining steps to install SuperUser and busybox on your EVO. If all goes well, your phone will be unlocked and rooted with Amon RA v2.3 recovery installed.
You will need to decide which features are important to you then decide which ROMs to try based on your personal preferences. If you like the HTC Sense overlay, I would recommend MikG 3.11 available at
MikG v3.11 [3/9/2012] for your consideration. This ROM has a Sense overlay that is fresher than the stock EVO 4G ROM but not dramatically different and comes with MikTweaks, a handy configuration utility that extends your ability to customize your phone.
Although you can use the stock HTC kernel, I would also recommend updating with a SBC kernel to allow the battery to charge to its full capacity overnight. I have installed the Classic Sense SBC v1.5 kernel available at
[KERNEL][WIP] Classic Sense Kernel v1.5 [JUL 22 2012][GINGERBREAD][SENSE] - xda-developers and have noticed significantly slower battery drain than I have experienced before. Be sure to download the SBC version for improved battery charging.
This YouTube video
How to Flash a ROM on the HTC Evo 4G.flv - YouTube walks you through the process of flashing a new ROM with Amon RA v2.3 recovery.
Always be sure to make a Nandroid backup from recovery before you start changing your phone's ROM or kernel for protection in case your phone fails to boot or becomes unstable after your changes. If anything goes wrong with the changes you make, you can boot into recovery and restore your working configuration and data from the backup you made earlier. Also make sure that your phone's battery has at least a 40% charge before attempting to flash the ROM.