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.