I broke my 5th metacarpal. What can I do?

See a hand surgeon. A 5th metacarpal fracture is a relatively common fracture. Certain features of the fracture may dictate whether something needs to be done or not. At a bare minimum, it should be splinted to allow it to heal. If it is angulated or causes some shortening of the bone, you may need surgery to stabilize the bone while it heals. See a hand surgeon to get it evaluated.
Get an x-ray. It is important to make sure the bone is properly alligned and allowed to heal. This is best done by your orthopedic doc or your hand specialist.