Get a credit card. Make one small charge that reoccurs every month. I have a subscription to gamefly. Pay that off every month. Watch your credit score go up. Don't get a card with a monthly fee. Just get one with a high interest rate. Because you're going to pay it off every month, the interest rate doesn't matter.
When I was eighteen, I bought a sewing machine at Singers store and they financed it by giving me a store credit card. I paid $20 dollars more a month than what they asked, paying it off in 8 months instead of 12. I then applied for a Gas card, gassing my car monthly and paying off the bill 10 days before the due date. After that, I started getting numerous applications for credit. Just be careful not to go overboard. Always ask yourself 'can I live without it' and charge what you know you pay off in a month. But for bigger purchases, try to pay it off within ten months to keep good credit. And above all, do not get too many cards. That would limit credit you need to buy a house. If so, close them and keep one main card. And then apply for credit to buy a house.