New Frontier, may as well start at the beginning. There's only 12 or 13 books so far....I can't imagine why you *wouldn't* start at the beginning for something like that. What's the advantage of starting later?
TNG doesn't really matter, although the A Time To series would do as well as anything.
Not to nitpick, but there are actually 20 NF books including the MU novel. But if you want to start the series, azzurri08, you shouldn't let that scare you away because they are really good.
As for the current TNG book line (which are the ones with Worf as E-E XO) you can pretty much star at any point. But if you want to go through the whole pre/post Nemisis series, then the A Time to... boks would be a good starting point.