Best to start with boots+3 pots or a doran's if you have a healing support
Interchange various items to your desires if you need it, like getting a Guardian Angel or some survivability items if the situation demands it. I tend to not need them because W and R really helps you get out of sticky situations easily.

Pros:
Incredible mobility with Rocket Jump (W)
Stupid range at level 18, (700+)
Buster Shot (R) is great for getting those pesky Vi's off your ***
Rapid Fire (Q) Attack steroid is godly, 90% at level 5, with your Hurricane and BoRK you will be a bite sized turret of doom with 2.5 AS.
Cons:
Abilities don't stack off AD
None really other than that, which really doesn't matter as you will most likely be too far away one shotting people to use your abilities.

Passive: Draw a bead
Tristana's attack range and Explosive Shot's cast range increases by 9 every level (not including level 1). At level 18, the bonus is 153 (703 range at level 18).
This passive is an absolute beast. If you manage to get to late game you've pretty much won.
Useful to note, you outrage Caitlyn at level 12

Rapid Fire (Q):
ACTIVE: Increases Tristana's attack speed for 7 seconds.
Bloody nice Q, means Tristana doesn't need to rush AS, and it's rather strong at early levels so you don't need to put all of your points into it.

Rocket Jump (W):
ACTIVE: Tristana fires at the ground to propel herself to a target location, dealing magic damage and slowing surrounding units by 60% for 2.5 seconds when she lands. On kills or assists, Rocket Jump's cooldown resets.
Makes you stupidly mobile, the cooldown reseting on kills gives you great potential for level one doubles, and makes tower diving almost safe.

Explosive Shot (E):
PASSIVE: Enemies explode when slain by Tristana's basic attacks, dealing magic damage to nearby enemies.

ACTIVE: Explosive Shot rends the target enemy, reducing healing and health regeneration by 50% and dealing magic damage over 5 seconds.

Gotta love this ability, it really helps when clearing out large waves and dealing with those pesky Mundos and Warwicks.

Buster Shot (R):
ACTIVE: Tristana fires a massive cannonball at an enemy unit, dealing magic damage and knocking them and surrounding units back.

Absolutely wonderful ult, large range and knocks people back in an AOE along with doing massive damage at early levels.

Focus on last hitting, try if you can to proc your E passive for more CS, and E+auto attack makes for some great harass as E range scales with your passive.

Not gonna lie, focus on last hitting early game, stay near a tanky person who can peel mid-late, and when a bruiser cluster**** appears, right click and get a penta. If any people try and melee you, W to a position closer to a peeler and make sure you are still in range of someone to murder. Also R then immediately W makes for an incredible getaway combo.