How do you teach a 7 year old to pick up thangs after themselves?

Encourage. Many children need a "reason" to cooperate. Give him encouragement- "when you pick up, we can leave to go to the store". Give him specific instructions- "let's put all the trash in here, then we'll put the clothes in the hamper". Give positive comments- "i like how you lined up the toys on the shelf. It looks great". It takes time, but they will start to see the benefit of their cooperation.