What the association will provide is soooo different from assn to assn.
Concerning equipment, buy yourself a starter package, such as this one:
http://www.honigs.com/detail.asp?Cat=1&Sub=97&Item=682
Honig's offer four different starter packages: With/without shoes and longsleeve/jacket (called cold weather) and longsleeve/shortsleeve (called warm weather).
I absolutely recommend the warm weather version. You'll get to use the LS-shirt a lot more than the jacket. Some assns. don't even allow jackets.
As with everything else, there are different shirts, pants and so on, but even the stuff in the starter package can be used for years, if you take care of it. Please note that the standard whistle you get in the starter package sucks (it's supposed to blow, sorry, couldn't helpt it...)
In many states, shorts are used for JV games and below. Some use white shorts, some use black shorts. Some wear short plain white socks with shorts, some wear long white socks and some wear the normal football sock with the top slightly folded down. Ask what they use in your assn. so you have the time to get the right type.
Try to find out early if you will be given rule books and mechanics manuals. Will you be working with NF (high school rules), or are you in a state where NCAA rules are used? There are different mechanics as well.
If you are serious about learning the rules, you should absolutely get this book here:
NCAA:
http://www.honigs.com/detail.asp?Cat=2&Sub=33&Item=560
NF:
http://www.honigs.com/detail.asp?Cat=2&Sub=33&Item=561
Apart from the rule book itself, this is the best rules publication you will find. Get it!
What's it like on the field for a rookie? Confusing. Don't be dissapointed if you feel that you didn't see squat your first couple of games.
My playing experience was rather limited when I started, so I didn't have that to fall back on. So I decided I had to learn the rules and let that be my "foundation". I hit the books hard, and studied like crazy. In my first season, I must have set the record for number of flags thrown. My feel for the game needed improving, and this is something that will come with time (and effort), if you are lacking in this department, like I was.
However, the time I put into rules study has really paid off, and I'm happy I did it. Knowing the rules makes you feel secure on the field, it makes you feel that you can handle stuff. When you feel secure, you look secure, and that will help you getting other people to accept your calls.
Don't be afraid to ask questions. The rulebook is tricky, and there is a lot more to officiating than knowing the rules. You'll find out that even though we have the same rules and mechanics, there are a lot of different philosophies (spelling?) and interpretations out there. Listen to what the experienced guys have to say, but at the same time, know that not everything they say will be the gospel. Some old-timers are so set in their ways, and officiating, like everything else, changes with time.
Post questions at boards like these, but keep the above mentioned things in mind when reading the answers. After some time, you'll know who you can trust on these boards.
Lastly (for this time at least) KNOW (and I do mean know) all the definitions in the rule book!!!! It is the KEY to understanding the rules.
Good luck and welcome to our fraternity!