Braces came off today and now I have a temporary retainer. Anyone got advice for wearing it?

In the first place, I notice that I kind of lisp now when I wear it. Does that go away eventually or will I get used to it? Do I have it in right? Is it supposed to fit snugly against the roof of my mouth or is there supposed to be a little tiny space? Also, I'm feeling a lot of pressure on my teeth due to the retainer. Is it moving my teeth? I thought a retainer was just supposed to hold them in place until they were set. Thanks!