I have not enough items, and after seeing how Too Many Items is better, I want to remove it. I open my MC Jar File, and I have no clue on what to remove. What do I need to remove so I can input Too Many Items?
There is no easy uninstall procedure. You need to know which files were in the version of "not enough items" that you installed and remove those manually.
To do that, you have to re-download the "not enough items" version that you initially installed and look at the files in its archive and match those with the ones in the current minecraft.jar, remove them there.
If you do not have an extreme amount of other plugins installed however, the easier way might be to simply re-download the minecraft.jar by clicking "Force update" in the options of the launcher and then install whatever plugins you need.
Open the launcher, click options, and click force update. Note that this will purge your Minecraft.jar, leaving you with a vanilla game. Consider looking into the Magic Launcher, which allows for easy installation and uninstallation of mods.
It has come to my attention that Not Enough Items uses Forge Mod Loader (FML). You should remove
NotEnoughItems.jar from the coremods folder in your .minecraft folder. Alternatively, back up your saves and nuke your .minecraft folder.