Song structure

Ive been playing guitar on an intermediate level for a few years and id like to get some pointers on song structure. Ever since my radio broke in my car I started humming and making up catchy melodies and songs as I drove on the highway to work. Ive been able to pick out the scale and figure out the major scale chords that I believe I need to use to create the rhythm. My problem is I cant get the right sounding strumming pattern of the chord patterns im humming. I feel as if there are more chords that I just dont know about. It sucks having songs in your head that you cant play for anybody. I thought that If I found the chords for the key that I picked out note by note when humming, id be able to figure out rhythm chords by ear to further construct my song. If anybody has any advice please reply. Thx.