Custom roms are a great way to change how your device preforms or looks. The most popular ROM is cyanogenmod. It is open source and for a lot of android phones. You can also search google for "Your Phone, Rom". It's not the safest thing do however because a lot of roms don't have sound or some other weird thing working. The best way to install new roms is to download Rom Manager. In this app you can download roms for your phone and install them easy!

As you can see, you can install ROMs from SD cards. If you find a ROM on the internet, drag it to your SD card and install it here. If you don't have one, you can download ROMs from the app!

Make sure you backup your current ROM before installing a new one in case you don't like it! When installing a new ROM it wipes everything on your device including apps and contacts!  You can always go back by clicking manage and restore backups.