If the customs officer admits you to the United States on visa waiver (WT/WB status) you need leave the United States within 90 days or less. In addition to that, you cannot change your visa waiver status to any other non-immigrant status without departing the United States (e.g. you cannot switch to H1B within America while on WT/WB).
You can change your visa waiver status to another status without departing the USA. My ex wife was from Italy, she came here on vwp and we got married. She stayed during the AOS process.
