Pros

+ Longest Range in Endgame
+ Hugh Damage output and attackspeed
+ No expensive build to be strong
+ W + R + Blink + Barrier = Good Sustain
+ Kills and Assists reset Tristanas Jump
+ Firefighter - Tristana = Best Skin

Cons

- Short range in Earlygame
- Low basic attack
- Need some items to be strong
- Squishy as hell
- Long cooldown on W and R
- Boring dance

Similar to the Masteries you have offense and defense runes. Quintessence and Marks are premanent attack damage for huge output from the beginning. With you start items Long Sword , Masteries and Runes you have 50+28 normal attack. This equalise Tristanas low basic attack.
Glyphs and Seals are for defense. Armor seals are a good answer for strong early AD-Carries like Miss Fortune. Magic Resist per level is good for your endgame. At the normal botlane there is few magical damage so you need the defense against it after the laning phase and at the end.

Starting with a Long Sword is in my Opinion the best. You have more damage and with 2x Heal potions you also get some Hp-Reg.
At the first back, be sure you have enough money to buy Boots for more sustain, Vampiric Scepter for Lifeleach and Avarice Blade for getting Gold. You dont need to buy it, if you dont like this item, but i think it is good becauce of the additional goldadvantage.
Your Core-itembuild is Statikk Shiv , Blade of the Ruined King and Berserker´s Greaves . With those you blow everything. With BotRK you have enough life steal to kill the other AD-Carry. Statikk Shiv get yourself into a farming machine. Passive E + Passiv Statikk shred minion waves like hell. With your Q plus the Crit and attackspeed from Statikk and the attackspeed from BotRK you do 1000 critical hits.
At the end you buy Bloodthirster first, because of the life steal. After it you buy Warmog´s Armor for more sustain and for being not that squishy. Infinity Edge is the best lategame item for tristana. Additonal attack, crit and stronger crits...everything you need.
Your coreitems aren´t that expensive and with an early Avarice Blade and a good farm, you should have it at the 25 minutenmark. Henceforward you kill everyone. Enjoy =)

At first you skill the Rocket Jump for escaping Ganks or to invade the enemy jungle by yourself. Maxing the Explosive Shot is good for the Laning-Phase by farming Creeps and harassing the Opponents. Rapid Fire is an Endgame-Skill. At the end you have lots of damage and range, so attack speed is the only thing you need. Your can use your Ultimante Buster Shot in a very flexible way. On the one hand you can use it in a defensiv way to save allies and push enemies away. At the other hand you can use it in a offense way to shoot enemies into the turret, to execute or to shoot them right into skill-shots or bad positions.

The Masteries are still self-explanatory.
Get the left site of Offense for huge damage output.
Important is the Armor-Penetration you get with Sunder and Weapon Expertise .
Frenzy is also nice for this Tristana build, because you gain attackspeed for crits!
Skill the defense tree to have more sustain at the early game. Hardiness , Durability and Veteran´s Scars give you Armor and Life to have an advantage over the other AD-Carry. Summoner´s Resolve lets your Barrier get even stronger!