As long as you have no criminal convictions, you will get a visa waiver form on the plane, together with a customs form. Just fill them both out and hand them in at immigration, no problem.
And if you have a record, then search on the 101 threads here that have discussed that specific issue lol.
p.s. It was 78F here in mid-March. Just been out this afternoon and it is snowing !? You might want to check a 5 day forecast (www.weather.com) just before you fly, and pack accordingly. Have fun!