The best thing I ever did with my kids, when they were little, was this. I walked into their rooms (this happened with more than one kid

!) with a huge black garbage bag, This was after several times of being told to clean up their toys. They were told this was their final chance, and after 15 minutes, *I* was going to clean up their room and donate the toys I picked up to children in need.
Worked like a charm

My daughter was easy, she picked up the toys. My son, on the other hand, didn't believe I was serious and said "SO! Go ahead and get rid of my toys, I'll just get new ones from Santa and grandma" So I put them all in a garbage bag, put him and the bag into the car, and dropped the stuff off at Goodwill. It wasn't all his toys, dont' worry

He screamed and cried for a while, but you bet your sweet booty that next time I asked him to pick up the toys, he DID
Nip those tantrums in the bud right now. If you don't you'll be dealing with it forever and ever