Do you feel lonely (孤独的) when your best friend isn't around? Maybe because he or she moved to a different school or a different class, and you want to make new friends. But how do you find them? Here's my opinion.
First, try by yourself. Start by looking around your classroom—think about which kids you'd like to play with after class. Say hi to them, and be friendly. Share something or invite someone to play with you. Say "Do you want to sit here?" in the lunchroom. When you are at break, just walk over to the kids and say "Hi, may I join you?" or just join in.
Then, ask your teacher for help. If you have problems with this, you can ask your teacher to help you make new friends. Teachers are usually very good at it. But you do remember, the best way to make friends is to be a friend. Being friendly, being helpful and sharing—pretty soon, you'll have one, or two, or even more new friends.
Maybe you will still miss your old friend sometimes. But when you see each other again, you can share something you didn't have before he or she left: You can introduce (介绍) him or her to your new friends!