If you don't know where to vote, googling "polling places" will give you a convenient google-based web form to find your polling location.
Make sure you know what initiatives will be on the ballot in your state - I've heard "but I didn't know that's what Prop #X did! I didn't mean to vote that way!" way too many times.
Know who's up for what spots and why you're voting for them. If you're not sure who to vote for, find an organization that supports things you believe in, whether that's Planned Parenthood, HRC, or whatever else, and see who and what they endorse.
Know your rights as they relate to your job and your ability to vote. Know the hours that your polling location is open (generally 6AM to 9PM).
Finally, vote, goddammit. It's both your right and your obligation as an American.
And if you don't vote, and try to complain to me about "what's happening to this country" any time between now and the next time you vote? I will slap you across the face.