I'll help ya out bro, I don't mind people asking repeat questions as much as others do here. For future reference, there's a search option on the main page and it will get you answers a lot more quickly than posting and waiting.
1. The easiest and best way is
iPhoneRingToneMaker, but it costs $15 currently. It doesn't require you to hack your phone either, which is nice.
2. Your next best option is
iBrickr, which will hack your iPhone for you so quietly you won't even notice it. It's free and it does ringtones almost as well as iPhoneRingToneMaker, but also lets you install new Applications, like the Nintendo Emulator and eBook reader.
3. The third way is through a program called
Song Sender that you put on your iPhone as a third party app, but it's a little more difficult to figure out. You install it through iBrickr, but once it's on you can just pick any one of your MP3s right from your phone and make them your ringtone instantly (you can't select which part of the song though, so to me this isn't as good).
Since you're a beginner, I recommend one of the first two options for now.