Start off real slow with really dumb songs....
Row Row Row your Boat etc......
Separate your thoguht process to so you know the guitar player so automatically, that the vocals are easy.

Do 'air guitar' to a well known song until you know it back to front.... then learn how it's played and develop your own style gradually.

The best bet also, is to take CD's and try to figure out how the guitarist is playing.