Beginner coming from (baritone) saxophone: What do I need to know?

So, I've always wanted to play sousaphone, and I finally am able to make that a reality. However, I don't have the money for a teacher or anything. I do have my friend's mouthpieces from high school, a jupiter AW24, and a helleburg (not sure of the brand, think it came with a miraphone), although I don't currently know where that one is. Someone in the community band I play in lent me a Conn 14k 3 valve.

Does anyone have any (free) resources they recommend. And maybe different mouthpieces? I want to learn new orleans tuba, so what would be good for that?

And, should my cheeks be tight or loose?