Meg Cabot's books are worth a look - the 'Princess Diaries', 'Missing' and 'Mediator' books are all good stuff. Typically girly stuff, but they address teen issues and concerns very well and not too heavily. The later books in all of these series see the main characters turning fifteen and sixteen and thinking about (but not actually doing) sex. Saying that, the last of the Missing series does see the character at around nineteen and sleeping with her boyfriend, although it's not gratuitous.
Another of my faves is Louise Rennison's 'Confessions of Georgia Nicholson' stories. Very daft and very girly, but very funny too (my hubby loves them). Start off there with 'Angus, Thongs and Full Frontal Snogging'.
Similar to Rennison's work is that of Echo Freer, who's written three hilarious books about one Magenta Orange, a very clumsy and accident prone young lady who means well.