Not Updated For Current Season
I just LOVE Tristana. She's cute, she has fun lines such as "Is that a rocket in your pocket?" and she has a HUGE cannon. What do you want more? This tiny megling can be a huge asset for the team as well as a great feeder/burden on the opposite side of the spectrum because of her signature skills; Rocket Jump and Buster Shot.
Being a big DPS girl, Tristana is also easy to whittle down. But in a good position, she can obliterate a good part of the enemy team and win team fights while also enjoying the explosions. As the game goes on, Tristana might either become a menace to the enemy team or an easily killable whimp.
In this guide, I will not be like "YOU, YES, YOU, PLACE ITEM/RUNE/MASTERY HERE" (/heavy) but rather suggest a direction to go in as Tristana's build is situational. I am also a rather public player, therefore my guide might be of some help but not entirely viable at higher levels.
Early Game: You should be either middle or with a teammate top or bot. Your objective is to get as many creep kills as possible while also harassing the enemy champion, preferably the one without any constant regen items if you are in a lane with 2 enemy champions. You should be able to get a champion kill or a lot of creep kills before level 6, at level 6, a hero kill is almost assured if you execute it well(Especially on a dual-lane).
Note: When soloing mid, there are some heroes, like Ryze and Teemo, that should not be underestimated. Make sure to know that they are capable of and most importantly, know what Summoner Spells they have to not be surprised.
Mid Game: Keep ganking and pushing towers in groups. I've got nothing more to say here. Don't be a kamikaze either, Tristana might have a strong attack power but she is not a tank. Avoid getting killed at all costs, kill if you have the health to. If you can, get the lizard buff, it helps a lot. Do dragon if possible(Don't if enemy team has clairvoyance unless you have rally), get your strong items as soon as possible and KEEP PUSHING IT.
Late Game: This is where Tristana starts to be reallllyy squishy(Especially against casters, fed heroes and DPS heroes). In big teamfights, you might want to go around and Buster Shot the enemies in the back towards your allies and flee/Rocket Jump towards your allies while your allies rush in(Which should be do-able with Cleanse+Rocket Jump), a good initiation is the key to winning a teamfight, or you can simply harass the enemies with your (now) long ranged attacks. Don't be afraid to use Buster Shot and retreat if things go bad, a dead Tristana is not a useful Tristana. Most important thing: Do not try to fight alone, you need your teammates to distract/disable the enemies for you to have the few required seconds to blast each of their heroes in less than a few seconds with Rapid Fire.
The last thing I have to say is: Tristana loves when the enemy nexus explodes, so do I.