Soak the nozzle in paint thinner, and put a few drops down the stem and let it sit for a while. If the paint is dry in the nozzle, the thinner may not disolve the clump, and you may need to put a needle through the front to push out the clog.
If the paint can is full, you may want to bring it back to the store for a refund provided that the labelling hasn't changed. I've had to do that recently for a brand new can.
TazMan2000