Gifts are pretty usual. Think nice birthday present. Money is always appreciated. My dh got some really nice bottles of wine for his bar mitzvah (which we drank sometime around his 21st birthday... this was wine that needed to be aged) but that seems unusual.
Dress depends on the congregation but tends to be more formal (from what I've seen) than normal. Think "Sunday best" but with a Jewish flair (depending again on the congregation, you wouldn't want to wear sleeveless or a miniskirt). That's for the actual ceremony, which will (probably) be at a synogogue. For the party, you'll have to ask what they're expecting -- some b'nei mitzvah's get REALLY formal, whereas others are more like family parties.
In terms of wearing a kippot, most synogues don't expect non-Jewish guests to do so, but I did when I went to my friend's. (I later converted). I've always loved wearing kipot. Don't wear a tallis, tho.
There are some books available to help you through the ceremony. I can't think of any titles off the top of my head, but you should be able to google them pretty easily. Your library probably has some as well