The best place to look for custom ROMs after rooting is
XDA Devs. Make sure you do plenty of research beforehand to make sure everything works with your phone, because some ROMs work perfectly with some phones and die in the ass with others.
One of the most important things to check before choosing a ROM is whether it requires that you update your radio. It'll tell you in the thread if it does, and give you the instructions for installing it. It's a very simple process, identical to flashing the new ROM.