EDIT: below is assuming you have
MDK version of the bootloader.
Simplest guide is this:
http://wiki.cyanogenmod.org/w/Install_CM_for_jfltevzw
You'll need a firmware flashing tool (ODIN or Heimdall) to install on your computer first.
That guide for CyanogenMod, but you can follow any other guide and once you get to the point of having custom recovery (TWRP or ClockworkMod as the guide uses) it's easy to flash any current custom ROM from
XDA. CyanogenMod 12.1 (Android 5.1) is fairly vanilla but has some small extra features and things like theme support built in, so going for that or any ROM based on it is what I would do rather than getting pure AOSP or Play (though you can do that). Threads for different ROMs might have other additional specific instructions or details about how well the ROM is working (newer ROMs can be buggy), so be sure to read the full original posts, at least.
You'll have to flash the Google Apps (GApps) after you flash the ROM. They aren't bundled with most (all?) ROMs (for legal reasons, if I'm not mistaken).