need help

Do you have to know all the notes on the fretboard before you can start playing some riffs, and if not where does one start to learn what needs to be played. I am not good at playing riffs, and I never know where to start. My thinking and it is wrong most of the time. Is that you would play scales, and this does not work for me not sure why it doesn't but never the less it just doesn't work. So if anyone could help. (((((HELP)))))))))))).
tks Ray

if your just starting out i would suggest learnin riffs taken from other guitarists. It sounds like its just a case that your not familiar with playing yet, this is a stage that you just have to break through, its like anything else you try at first you don't know where to start but after awhile it will just start to click. If i were you i would stick on some led zeppelin and pick a riff you like, go get the tabs for it (if neccessary) and then start practising. thats pretty much what i did and it worked pretty well i think, and as Jimi Page is the Riff Master (undisputed) i think its a good place to start.

Well once you get the chord shapes and such down, find some simple riffs from artists you like... Easier to get better at stuff you like to play...