Go over to them ask them a few questions, Maybe (mattering how young you are) at recess if there playing a game ask to play with them or something you have to talk to them maybe ask them a question about homework or something, Hope I helped:)
You find out about them. Notice on their t-shirts what singers they like, or maybe their favorite color. Ask them questions. Always have something to do or something to talk about. If there's awkward silence they won't really want to hang around you because they're trying to get away from the awkward feeling. So make sure you can talk a lot and leave conversations open to them.
If you are in school and you see this person at recess or lunch, casually walk up to him/her and start a conversation by introducing yourself and asking him/her about a good movie you heard about, or joke about the cafeteria food, or just walk by and smile till you see him/her again and find there is something you really would like to talk about.