Hello, could somone try to explain harmonizing really simple, i've been trying to get a hold on it, but i don't really understand it.

Also, if someone who's good at it give an example on how you could harmonize this melody. And maybe describe how to do?

E I----------------------
B I----------------------
G I-12-14--9-10-9-10-9---
D I----------------------
A I----------------------
E I----------------------

With backing of simple G5, A5, E5 and F5 chords.
get a harmonizer?
My Gear:
Epiphone Classic Les Paul
Ibanez JS1000 w/ Seymour Duncan Jazz and Pearly Gates
Alverez Acoustic
Fender Hot Rod Deluxe
Boss GT-8

Add Me?
Well, we're two guitarists, and it's much cooler to do it on two peeps, and harmonizers cost money.
Probably that's going to sound G Mixolydian which is from the key of C.

So, use the notes of C, of which all the ones you gave are in. Then for every
melody note you want to harmonize, choose the note in the scale, after the next
note (i.e. every other note). That's one way to do it that will likely sound ok.