A good rule of thumb is to sell items for ten cents on the dollar. That's pretty much what people are willing to pay. If it's a big, expensive item, you can raise the price a bit, with the understanding that someone may want to negotiate a lower price.
You could also set up a "$1.00 Table", where everything is a dollar. Be SURE to mark everything otherwise. People generally won't buy an item if it's not clearly priced.
Also, carry your cash in a fanny pack that is with you all the time. You'll also need a permit. They're free where I live, but in some cities you might have to pay.
Make sure you check with your city about where and how many signs you're allowed to place. Make sure people know where you are.
I hope you have a great first yard sale. It's something we do about once a year and now my kids are at the age where they pretty much run it for me! Of course, they get a cut of the profits:)
D. L