For a traveler, the US Customs and Border protection agency is and can seem like a hindrance or intimidating process.
While it's not really a pleasant experience.. very regimented and rigid.. it's a very necessary "evil" as it were.
There is a process to follow, and if you have nothing to hide, or no reason to have issues, you should be fine. My recent trip through customs was my first alone (without the parents) and first in at least 15 years... and thus certainly first since the day that will always live in infamy for all of us..9/11.
My girlfriend and I followed the steps laid out for us (and even were treated well and greeted with some smiles when we had a few questions regarding the process)
Just do as you are told, and be honest and you won't have any problems. A great experience this time and on our way back through.