One of the lst things they ask us (l) How long are you staying in the US? Then,
(2) What date are you returning?
(3) What is the purpose of your visit?
Be truthfull. If you are visiting a 'pen pal' and his family, tell them. If they want further info they will ask for it.
They will always take your photo and fingerprints so the next time you enter the US, they will have your details on the system. Never OVERSTAY. Don't return on the 90th day under the VWP. Planes get delayed sometimes for a day and, I believe they record when the plane successfully leaves the tarmac, not when you board at the Gate. I have gotten off the tarmac and been returned to the Gate. It's OK if you only travel occasionally but if you visit more often you need to watch the dates.

Most importantly, you have a return ticket.
Only answer the questions they ask, don't supply more detail than they need.
Have a good vacation.