"Im new to playing charge character sand im learning Boxer, how do you ultra or super after you juggle an opponent with the buffalo headbutt? Am i suppose to be charging during the headbutt or what?"
Yes. When you do buffalo headbutt, do the up with up-back, then move to down-back. this keeps your back charge, so when you land, just do forward back forward and 3K. Hold K for the first hit, let go for the second (so a straight comes out, missing the opponent and letting them fall a bit), then hold K for the rest of the ultra.
"this is just a general question about charge characters: whenever i play them, i seem to be extremely predictable... "he's backing up, here comes a sonic boom"
its not that way when others do it for some reason. how do you get less predictable"
You can charge back with down-back. Also, you can move forward with Guile while holding back by pressing back and light kick to do his knee bazooka. And don't be afraid to drop your charge, walk up/jump and use your normals a bit.