I have a small bump on my forehand. It has been there for a few months now. It's also hard (not soft). What are the possible diagnostics?

Cyst, tumour. If it is level of wrist it can be ganglion. However if it is back of hand it may be benign tumor. This can not be definitively evaluated by exam alone. See physician for possible biopsy and definitive diagnosis.
Gangion cyst? Ganglion cysts are small round "balloons" that project out of inflammed joints of the hand, fingers and wrist, usually on the top surface. They are filled with a very thick jelly-like substance that when packed into a small cyst feel very firm. Ganglions can be treated by draining them and injecting them with a steroid or by having a surgeon definitively remove them if recurrent or sore. Harmless.
Ganglion. If the mass is at the wrist level it is most likely a ganglion. These can be hard even though they are cystic. Surgery to remove it is simple if it is growing, painful, or restricts motion.