Read about rooting and unlocking boot loader , flashing a Rom, and updating your bootloader (modem). I like 4.1.2 Roms, Obsessed Rom, TKRom, Clean 7 Rom, and Synergy. Each have themes that are great. Read up on rooting, unlocking boot loader, flashing Rom and changing basebands BEFORE YOU DO ANYTHING ON YOUR DEVICE. Each carrier and model is not the same so make sure you only follow directions for your specific device model and carrier, if not you will most likely have a paperweight. Good Luck.
Lean kernel , KT747, and a few others have 4.1.2 kernels available.
Cruzin' tha Galaxy SIII
Verizon Samsung Galaxy sIII - ROM Synergy nightly (awesome)
Thunderbolt - ROM SynergyRom (WOW) Thanks Virus & Everyone
Incredible 2 - ROM ? Wife (changes mind daily)
Viewsonic G Tablet - ROM Vegan Ginger
retired HTC 6800 Alltel (Verizon)
Thanks Dev's & All Who help make our devices (toys) so cool.