Talk about it
Casual or serious, in some ways relationships all work the same. Skills you can build in your everyday relationships will help you in more serious relationships later.
Many people experience romantic relationships in their teens. But you should never feel like you have to do this. You are first and foremost a valuable and unique individual. People find themselves ready for the romantic kind of relationship at different times, if at all, and the smartest thing is to let it happen when you're ready, on your own terms, and not before. You should be more concerned about not getting into a serious romantic relationship too early.
Communication skills help keep you and the other person on the same wavelength. Conflict resolution skills, which are an essential part of your communication skills, enable your relationship to thrive despite the differences between you.
"I" statements and "you" statements
Use "I" statements to say how you feel, instead of "You" statements about the other person. Think about the difference between these two statements (imagine someone is saying them to you):
"You're always late! You don't care about anybody but yourself! You can't even get yourself out the door!"
"When I have to sit around waiting for you, I feel like you don't really want to be with me. I feel like it must not be very important to you."
Accept the other person the way he or she is -- unless, of course, the other person violates values that are important to you. In that case, you might need to withdraw from your relationship with the other person. If your girlfriend starts cutting classes, for example, and pressures you to join her, you might decide you can't be together so much anymore.
Complain, don't criticize. If you're the one with the complaint, don't make it about what's wrong with the other person. Make it about how it affects you and makes you feel. As in, When you do that, I feel this (and I don't like it). When the other person complains to you, listen and make sure you understand. Don't be defensive or evasive.