Buy little bullshyt you have to buy anyway like gas and groceries on a card, put any and all cash back on the balance, PAY IT OFF EVERY MONTH AND NEVER RUN A BALANCE.
NEVER USE THE CARD FOR ANYTHING YOU CAN'T AFFORD UNLESS IT IS AN ABSOLUTE EMERGENCY
Your score will increase itself over time. To speed it up, have one or two more cards and/or an auto loan and do the same and never miss a payment. Some credit bureaus will let you put utility bills on there so if you pay those on time that will improve your score. I use Experian's Boost program for that.