What do you have to do to expand your vocal range?

I am a b and I have a tendency to sing all the b parts in choir an octave to low. and i try and try to sing higher. but when i do it woulds sounding like this awful chord. There is two notes that im singing. but it sounds terrible. and i can rarely enter (or exit) my head voice with with out my voice cracking. and i can't even get into like a tenor's range. I tried to sing "learn to fly" by the foo fighters and some parts in there that are to high for me. so Can you help me and give me some practice techniques, pointers, links, explain why my voice does that damn chord thing, or anything to help i want to able to sing with a vast range, cause i want to anything and everything. Thank you all in advance.