Hey there. This build for Tristana is for late game domination. Early game, you need to farm. Harass with E if you wish, but be careful, and know that the fun doesn't really start until you reach level 18. Play it safe early, and be rewarded later.

Pros: Nearly unmatched damage.
75% lifesteal!
87% crit chance!
Carry your team to victory!

Cons:
Not a lot of health.
Poor positioning will get you killed quickly.
13% chance to not crit.
If you do poorly, the game may end before you reach your range potential (when you are most dangerous).

The runes here are for late game.
Armor Pen is good all game long, as you won't be buying any armor pen items. Early game they make a huge difference, and late game they're still ok, especially vs. those that dont' get amor items.
HP seals are so you have more hp later on, and don't get bursted down. You shouldn't being going 1v1 with bursting heroes, even late game, but if you do, the extra health is valuable to keep you alive.
MR/Lvl Glyphs for late game MR.
Armor Pen runes, for the same reason as armor pen marks.

We're going offense here, for obvious reasons, and 9 in utility for the ghost point and the experience. You need to get to level 18 ASAP, and xp materies will help you do it.

Your items are there to provide good damage, attack speed, and lifesteal throughout the game.
Get boots of speed first so you can poke around getting last hits with minimal risk, then straight to a bf sword. You need the damage.
Finish your boots (you need to be mobile and on the outside of a fight), and get your bloodthirster. Farm it up until it is maxxed, then get a zeal and finish your phantom dancer.
Inf. Edge next for passive and extra damage.
Another PD for AS, crit, and movement speed.
It should be very late game now, and get another bloodthirster so you can have 50% lifesteal and lot of damage.
Your next item should either be a Frozen Mallet if you NEED the hp. The passive and your PDs should help you keep up with enemies even without your boots. If you don't need the HP, get a third Bloodthirster for 75% lifesteal and a metric ****ton of damage.

We start off with Rocket Jump for safety. You need to stay alive at all costs. If you get super ganked at level 1 you can flash and rocket jump away, greatly decreasing your chance of dying.
Next, get E, then another E so you can harass with it and punish enemies that use heals or hp pots. You can also last hit and safely push the lane (if you wish). If you don't want to push, DON'T get any points in E, because you can't toggle it.
Alternate W and Q instead.
Get your ult at level 6, 11, and 16.
Max your Q by level 9.

There are a few options, and preference should decide what to get.
If you like pushing lanes a lot: Get teleport. It allows you to push a couple waves, then teleport to the other side of the map while your enemies try to gank you. You can then push the opposite side and make your enemies waste valuable time or get them out of position.
If you want kills, kills, kills, get ignite or exhaust. Ignite is good offensively against heroes like tryndamere, but exhaust also gives you defensive options.

The reason I think ghost and flash are the best, is because ghost allows you to kite enemies (even ranged ones!) because of your amazing range at level 18. Flash is a almost a must-have because of how useful flash + rocket jump is. Sometimes your W isn't enough to save you, but flash + W AND ghost? Only a teleporting enemy will be able to catch you.

Playing Tristana requires a certain amount of patience. I'm always itching for an early kill, but it can go very wrong, and many times it is not worth it to chase an enemy with rocket jump or bustershot someone early game. Just remind yourself that come endgame, only a tank will be strong enough to brave your range and damage, but early game your advantage is not as large. Wait for your passive to max your range, and hopefully your amazing creep score will give you the edge to out-item your opponents.