Here are two Tristana builds I would recommend. The first is more of a tanky, jump-in-and-kill type of Tristana. The second is more of a super carry Tristana, but it's harder to pull off.

Go with the standard 21/0/9 masteries. I don't think there are any good reasons why you wouldn't go at least 21 in the attack tree. You could move the other 9 to the defensive tree and take some armor, magic resistance, and dodge chance, but you shouldn't be taking many hits anyway since you're ranged and should be protected by your team.

I chose the standard armor penetration marks and quints for Tristana. Another option for these would be critical strike damage for very high late game damage. Then, I prefer mana regen seals and magic resist/level glyphs, to help you survive against burst mages late game.

I usually go double Doran's as Tristana. Of course, your start is usually situational, so do whatever the game calls for. Sometimes, you'll be able to get by with just one, while other games you might need three. Other good starts include Long Sword + Health Potion or Boots of Speed + Health Potions.

Berserker's Greaves are generally the best boots for Tristana. They're cheap, great for early game, and will increase your DPS. Other notable options for your boots include Boots of Speed and Mercury's Treads for tenacity.

- There are two reasons why you should get a Wriggle's Lantern: for life steal and for free wards. You might think this is a silly item, since you're a ranged carry and should maybe just go heavy on damage, but the life steal allows you to sit in your lane much longer and farm, the wards will keep you from ever getting ganked, and it's a pretty cheap item that shouldn't set you back far.

- Infinity Edge will boost your damage quite a bit and allow you to kill very fast.

- This is a great item, especially when combined with an Infinity Edge. The movement speed also allows you to chase and get away from fights quite easily.

This is the build for a more aggressive style of play or else when you feel you need to build some survivability.

- Frozen Mallet is a great item for Tristana, especially if you're going for an aggressive style of play like this build suggests. The health will help you stay alive in fights and the slow will allow you to chase very well. It gives a little damage as well.

- With Madred's Bloodrazor item, along with the rest of your build, you should have almost-maxed out attack speed when you use Q. With maxed attack speed, the Bloodrazor's passive will be dealing a significant amount of damage. This also means that your Frozen Mallet will be slowing 2.5 times per second, making you especially annoying to play against.

- This is an optional item at the end of the build. You can choose whether you want to sell your Wriggle's for it or not. Bloodthirster will give you more damage and life steal, but you won't have free wards anymore. You could also choose to get a Banshee's Veil or a Guardian Angel instead, if you need more survivability instead of more damage.

This is the build for a passive Tristana that waits for team fights to completely get going before you jump in and kill. This build is great for when you're fed and can supercarry or when you can safely hit people without having to worry about dying.

- The Bloodthirster gives you lots of damage and more life steal. The main reason you get this is just for lots more damage. Also, with two or more life steal items (the Wriggle's and this), you can safely hit enemies that have Thornmails, without killing yourself.

- Last Whisper is an extremely cost-effective item that will allow you to kill much faster, especially if they opposing team is stacking armor. If most of the other team isn't getting armor, you can choose to get a Black Cleaver or something else instead.

- You choose to either sell your Wriggle's Lantern or your Berserker's Greaves for your second Phantom Dancer. If you need movement speed, keep the boots. If you don't need the movement speed, you should normally choose to keep the Wriggle's. With two Phantom Dancers and no boots, you should have around 400 movement speed, which should be around the highest in the game. So, having no boots will hardly set you back at all.

- Like with the first build, this is optional. You can choose to keep the Wriggle's or boots or you can choose to sell it and buy the Bloodthirster. Getting the Bloodthirster will give you an extreme amount of damage.

Normally, the ranged carry should go bot lane with a support. Tristana also can make a decent mid or top though, as long as she isn't too aggressive and can stay alive while farming well. If the enemy ever gets too close to your turret, make sure to take advantage of the opportunity by jumping to the other side of them and ulting them into your turret. Then, use E and ignite on them and you should get an easy kill. If you're laning with someone with strong CC, especially a stun, this can be a lot easier to pull off. You can't do it if your lane is pushed though, so stay near the turret!

In my last game, I went 22-2-4 with the second build (yes, in a level 30 game). So, I do know what I'm talking about and these builds do work.