Getting your pet quests done can easily get you a level or two, depending on your questing style. Levels 11-19 introduce a lot of your skills. You'll want to be putting your talent points into Lethal and Mortal Shots.
Skills, Levels 11-19
- 12: Arcane Shot (Rank 2), Distracting Shot (Rank 1), Mend Pet (Rank 1), Wing Clip
- 14: Eagle Eye, Eyes of the Beast, Scare Beast (Rank 1)
- 16: Immolation Trap (Rank 1), Mongoose Bite (Rank 1), Raptor Strike (Rank 3), Aspect of the Cheetah
- 18: Aspect of the Hawk (Rank 2), Multi-Shot (Rank 1), Serpent Sting (Rank 3), Track Undead
The Beast Mastery skills you get at level 14 are interesting, and fun to fool around with, but most of the time have limited application. Scare Beast is the most useful; you can use it to temporarily shed an extra mob, but it really shines in PVP, against druids in a Feral forms, and to a lesser degree against shamans in Ghost Wolf form. This means fearing the flag runner in Warsong Gulch. Enjoy. ;)
The other two skills you get at level 14, Eagle Eye and Eyes of the Beast, are of less frequent utility: they're basically scouting skills. As I mentioned when I was talking about glyphs, Eyes of the Beast was basically used for pulling two bosses in Molten Core, as it lets you put your sight into your pet and control it directly. (Running around on your pet, if it has Dash, and hopping all over is, admittedly, kind of fun. It can also freak out newbies unfamiliar with hunters.) Eagle Eye is similar to the shaman Farsight - you click on a distant spot of land, and your point of view is transported there. You can change your camera angle, but not move it. This is another skill that can be useful in PVP, especially places like Arathi Basin, if you're trying to find a weakly defended node to assault. All in all, though, they're more fun, or for scouting, than readily combat-useful.
At level 15 you can equip your first glyphs, one major and one minor. I'd go with either the Serpent Sting or Mending glyph for the major slot, and the Mend Pet glyph for the minor slot.
Level 16's skills are somewhat more useful - you pick up your final melee special attack, Mongoose Bite (which can crit nicely at higher levels with a slow two-handed weapon), another rank of Raptor Strike, and your first trap, Immolation Trap. Immolation trap is a single-target trap that does fire damage over time (a DoT). It's a nice bit of extra damage that keeps ticking while you're, if needed, kiting. (I'll include my trap macros when we get a second trap.) You also get Aspect of the Cheetah, which you used to have to wait till level 20 for (as of patch 3.2 - so not quite yet, but soon). Cheetah is a nice speed boost, as you won't get a mount until level 20 (as of patch 3.2 - originally it was 40, then 30), assuming you can afford it. The major detractor is that if something attacks you, you're going to get dazed. It's only a 30% boost, not the 40% shamans and druids get, but it's still quite nice.
When you reach level 18, you pick up new ranks of Aspect of the Hawk and Serpent Sting, and you get to track Undead. No, this does not include the Forsaken - they're under Humanoid tracking with the rest of your fellow players. You also pick up Multi-Shot, which is your first multi-target ability. For a long time, when Aimed Shot had a 3 second cast, Multi-Shot was in its position in the full Marksman shot rotation. Now, though, with its higher mana cost and Aimed Shot benefiting from the same talents, save Multi for actual multiple-target situations - it's especially nice in PVP, as you don't have to worry about pulling aggro off something (although you're likely to have someone in your face, regardless).
If you're playing Horde and have the Burning Crusade, doing the quest chain in the Ghostlands that leads up to "The Traitor's Destruction" will land you a nice blue bow. There doesn't seem to be an equivalent ranged weapon from quests on the Alliance side. If you don't want to hit up the Ghostlands or, alas, are playing Alliance, there's a slightly better bow from Lord Serpentis in Wailing Caverns.