There are many phrases I need to learn before I go. I think there's an important one that means something like, "I am inconveniencing you by entering" but I can't remember it.
You're probably thinking of "ojyamashimasu," but this is typically only used for entering...I doubt you'd want to use it over and over. There's also "osewa ni narimasu," which basically means, I will be in your care. Change the narimasu to the past tense when you leave. There are little things like saying "o saki ni" when you plan to do something ahead of everyone else, i.e. bathe, eat, etc., but this kind of thing is usually best picked up as you go. As I'm sure was already said, each family is different and will treat you differently. I've had several encounters with host families, and most are very understanding about gaijin not knowing typical home behavior.
When presenting a gift, say "tsumaranai mono desu ga" - "it's nothing impressive but..." even if the thing costs a hundred dollars. If you want to impress, brush up on your keigo, but I think they'd be more shocked/delighted by it than expect it.
As far as typical manners...there's no harm in taking every precaution...regardless of what your host family may do (of course, until they become comfortable around you, it's unlikely they'll be rude either). I got to the point where I'd burp randomly around my host parents, come home at 7 am, etc., and they paid no mind because we'd gotten used to each other.
Finally, let them know what you're doing, plans for the day, etc. Chances are, they'll be going to pretty great lengths to prepare an extra meal and show you around. If you made plans, let them know ahead of time. Have fun.