Can someone please explain to me how to use one? Do I play a note everytime the metronome beats? Lol, i'm really confused on how to use one.
Well you can do, or you can do two for ever beat, or three, or four, or five, however many you want.
You just use it to keep you in time to help you with your practice and make sure your playing is tight.
Well there are basic rhythms in common music, Whole notes(which take up 4 beats(4 clicks on the metronome)) Half notes(2 beats(two clicks)) Quarter notes(1 beat( 1 click)) Triplets(3 per beat(3 per click)) Eighth notes( 2 per beat(2 per click)) Sixteenth Notes(4 per eat(4 per click)) Sixteenth triplets(6 notes per beat( 6 notes per click))

And you just play anoy of those according to how your metronome is set.
You practice to a certain speed. Know the note values in the riff you are playing and play them along to the metronome, at the speed you want.
Because you can practice something using it to keep the timing correct, and then work on your speed.
For example you could play something made up of 16th notes (4 notes per beat - 4 beats ever click) and practice that until you can play it cleanly. Then you can up the tempo a little bit and practice it at that tempo, then up it again and so on.
Then you should be able to play it a lot faster after a lot of practice and still play it tightly.