Say that someone "is supposed to" do something if:

• there's a rule about it
  

  I think we're supposed to shower before going in the pool.

• there's a custom that says something about it
  

  You're supposed to take your shoes off when you go into someone's house in Japan.

• you have already agreed to do it
  

  I'm supposed to pick Janelle up from school this afternoon. 

Note that "supposed to" can also be used in other situations, like when you're reporting information that you heard from other people.