There are a number of great ways to build credit; it depends on what your timeframe is and what purpose you have for building credit. Are you trying to get lower interest rates on credit cards? Or are you trying to get approved for a mortgage?
One way to build credit is to be an "authorized signer" on a credit account. This can be a bank loan, a credit card, or a consumer account. What is important is that whomever adds you as a user is a responsible person and pays their bills on time. In order to build credit you MUST make consistent, on-time payments to your credit accounts.
Another way to build credit is to invest in a secured credit card. This typically requires a deposit that reflects your credit balance. Use the card once or twice a month, but do not exceed more than 25-33% of the card balance and be sure to pay it in full each month. As you carry a balance you accrue interest and it can lower your credit score.
There are some alternative ways to build credit, as well. A few companies offer installment accounts that allow consumers to make payments on merchandise and receive credit history each month. I work for a company that leases laptops to people and reports credit history to all three credit bureaus.
My advice is to set some goals for yourself, do a little bit of research and cost comparison, and if you can afford it, find a financial adviser. I would recommend only applying for the types of credit you are actually interested in; too many inquiries can be a negative factor. And read all of the fine print! This will save you potential frustration when paying creditors back.
Good luck with building your credit! :)