As Pocketbook Pup said, you will get a form on the plane to fill out. On the front there is a space for you to declare the total amount of all items purchased abroad, and on the back you can itemize things if you want to. You are supposed to declare everything. US citizens get an exemption of $800 per person, so if you are traveling with your spouse, you would get an exemption of $1600 for all your purchases, then you would technically be required to pay import duty on the rest.
That said, in my personal experience, particularly at a busy large international airport, many customs officers don't want to bother with the paperwork. Import duty rates are pretty low for the first few thousand dollars over the exemption. I always honestly declare my purchases and we almost always exceed the exemption and have never been required to pay import duties. (I am probably tempting fate and will probably get stopped next time now!)