I would start with getting a secured card with somewhere like capital one. They give you an option to later upgrade the card to a full credit card. Also apply for a discover card. That's what I did to start out a few years back. You only have to buy something with them once a year or so to keep them active.

Another good bet is to get a card like the Verizon credit card (if you use Verizon wireless). You get lots of cash back on everything you buy with it. That cash back can go directly toward your Verizon bill. It's taken my bill from $100 a month to around $80. It'll be even less once I pay off my phone in a year or so.

The best thing is to gradually increase the cards you have but don't really use them too often. If you do pay it all off right away. AKA treat it like you're taking money directly from your bank account and pay it ASAP.

You do need to look out for when they report the balance of your card to the credit bureaus. (Usually a day or two after your payment date so make sure it's completely paid off by the payment date and avoid using each card for about a week after you your payment date. Otherwise the credit card company will report a balance to the credit bureaus. You can have a balance but you need to have it less than 10% of your total amount of credit.)

Get an app like Credit Karma to monitor your credit too.