...electric guitar? because a lot of the songs I learn have that..
in case it matters, i run my guitar through an RP100 into my amp.. does it change it?

face the guitar pickups ( close to the speaker).

Finding the right spot near the speakers will do it. If you're good at it, you can even control it.

In the Beatles' song "I Feel Fine", there's that feedback at the beginning. Originally, it was an accident but John learned how to control it so that he was able to get the exact same sound whenever he did the song.

Well, you can shove your guitar into your speaker, but I think what you're actually hearing in a lot of songs is harmonic overtones with high gain. I think you'll hear a lot more pinch harmonics than you'll hear feedback from facing the guitar into the speakers. Feedback is easy though, like they said, just aim the pickups at the speaker.