You can use it when you should do something, when you're expected to do that thing. For example, if you're a student, you can say: "I'm supposed to study this afternoon, but I think I'll play videogames".

Hello, We use this phrase in different ways. Generally, it introduces something that the speaker considers a bit hypothetical. That's why it is sometimes followed with "but". "Supposed to" is very similar to "should" or "(have to). It can connote that the speaker does not believe or agree with the statement. Ex: This soup is supposed to be served cold, but I like it hot. We're supposed to do our homework first, then have a snack.(but we don't always do that) You were supposed to clean your room, not watch TV. (but you watched TV instead) I can't go with you to the movies. I'm supposed to wash the car this afternoon. The repair was supposed to be done already. Call and check on it. What's wrong with the car? It's not supposed to make that noise. It can also express something that people say is going to happen or to be. It reflects uncertainty Ex: It's supposed to rain tomorrow. Let's plan to golf another day. It was supposed to be a sunny day, but it rained all day. There's supposed to be some more examples in the back of the book. (I can't confirm that there are more examples, I am just saying that there are "supposed to be") It's not generally used in a future tense. At least I can't think of any examples. Hope that helps a bit! Martha

When you are speaking about something that you think to know how should be going, like something already planned. For example "I suppose I should get going"

That would be grammatically incorrect. It is 'supposed to be'.