La Bamba - Ritchie Valens
This is an all time classic riff for guitar, and everyone should know it. It's not hard to play, sounds pretty cool, everybody knows it and it transforms you C, F and G into a great tune!
And it's fun! :)
Write it out in your own tab if you struggle to get it from the video - just use the blank tab. The rhythm is as follow - with the first bit being the pick up and the 1 in brackets has no note on it:
+4+ | 1 2 + 3 4 + | (1) + 2 + 3 + 4 + |