I have a 96 Cavalier and I can put my key into the locks, but it won't turn. The passenger side door just froze up this morning. It's almost like the locks are frozen but they don't even work in 30 degree weather! Any suggestions on what I could do? Or do I have to take it to a mechanic?

Inspect your key closely, it may have got bent or twisted, even a little bend or twist can keep it from working, also have a copy of the key made and see if it works.

also , my passenger side door is stiff to use and my trunk doesnt work, its possible their seized

It is a possiblty that they are seized but doubtful its very possible that your key has just wore down so much that it wont turn in the slot so just making a copy wont fix it because it will copy the same worn down key so my suggestion would be to goto a chevy dealer and have a new key ordered they can do this for you by giving them your vin # this will probably cost around 15 bux i had to do this before on my 91 cutlass supreme After that it worked perfect