"Suppose" is used to introduce a suggestion:
Suppose we get lost. (What if we get lost.) - "Suppose" is used in place of "What if."
"Supposed to" is used to talk about what someone generally believes is true.
These chemicals are supposed to kill cockroaches.
You're supposed to be at work right now.
- "be" + supposed + infinitive
You "are" + supposed + to be........
The difference between your two sentences:
I suppose that
It is supposed to be
-The first sentence, "I suppose that," introduces a suggestion. The speaker is speculating.
-The second sentence, "It is supposed to be," comments on what the speaker or the public generally believe is true.
I suppose that this explanation might be wrong, but
it is supposed to be true, according to Michael Swan's book Practical English Usage.
I hope that helped some.
The other answers above were also good, fyi.