I fell full weight on my steps and hit my forearm on the corner it cut my arm and now there's a huge lump and bruise and pain?

GO TO ER OR DOCTOR. You have an injury to your forearm with a lump, pain and a cut.It needs to be checked and may need xrays to rule out fracture and cut may need medical attention as well, and you may need tetanus shot if it has been ten years since last tetanus shot.
Sounds like. You really hurt your arm! ouch!. Does not sound like a break, but a hematoma is the name of the huge lump from swelling and surrounding bruise. It hurts a lot. You should have cleansed the cut well with soap and lots of water. Is your tetanus shot up to date? Put ice on the lump (frozen peas or corn works great) 15 minutes 3 times a day) for 3 days to help the swelling. Antibiotic cream for cut.