Your skills with each weapon unlock through use. You start with one skill, and then you unlock #2-5 by killing 5, 10, 15, and 20 enemies. (It counts as a “kill” if you do any damage, so hit those events as AE away.) One-handed weapons only have #2 and 3, off-hand items are always #4 and 5, and two-handed weapons use all five slots. For those off-hand items, you advance them if you already have #2 and 3 done on your main hand.
An Engineer will do this without noticing. Pistols can be used in either hand, a shield in the off-hand, and rifles and (underwater) harpoon guns are two-handers. 50+50+50+35=185 kills to unlock everything, 50 of which must be underwater. Don’t worry, Engineers have lots of options because they get kits in their utility skill slots. A Warrior will be spending a bit more time even collecting weapons, with 11 weapons available in 21 combinations. That’s 520 kills. The Elementalist has just 5 weapons, the second-fewest, but remember that I said each weapon has four elements. An Elementalist needs 200 kills per attunement to unlock everything, so 800 kills, 200 of which must be underwater. Elementalists gets nice AE skills, so you could spend time in crowded events and unlock many of those quickly, but 800 remains a large number.
In the long run of the live game, you need to do that once ever, and you will certainly kill more than 800 enemies over the life of a character. Still, they’re looking at that:
This is definitely a problem that we would like to address. While it only happens once per play through on a given character it still feels a bit unwieldy on Elementalist.
— Jon Peters (ArenaNet Game Designer), Guild Wars 2 beta forums
In GW2 BWE1, I tried an Engineer and an Elementalist. You notice that difference.