The bright primary colors in which the plot, dialog and characters of this movie are cast gives it away early on: this is a cross between a soap opera and a sitcom, made purely to entertain. As such, it's actually pretty good.
Mandy Moore is adorable. She seems to be learning how to act as she goes along, but isn't that how most of them did it? Give her a few more years and some better scripts to work with and she could be a major star.
The real problem is that for the movie's target audience of middle-class suburban white teenagers it's positively overflowing with groaners -- embarrassing "banter" between the kids, cartoonish characters (idiotic philandering husbands, evil boy-stealing girlfriends), and a preposterous storyline. My own teenage daughter and her friends thought How to Deal was, and I quote, "stupid." But for us middle-aged parents nostalgic for a time in their lives they've almost completely forgotten, it really isn't that bad. Give it a chance.