Hello! Since this is my first guide I have no idea how to make one. But I've seen many horrible Tristana players and I would like to share my personal build with everyone. The build changes depending on the other team as well as your own. I'll do my best to explain the reasons behind my build.

Personally I don't think runes make much of a difference. But the best runes in my opinion to get are:
AD Build
Greater Quintessence of Desolationx3
Greater Mark of Desolationx9
Greater Seal of Attack Speedx9
Greater Glyph of Attack Speedx9

AP Build
Greater Quintessence of Magic Penetrationx3
Greater Mark of Magic Penetrationx9
Greater Seal of Cooldown Reductionx9
Greater Glyph of Scaling Cooldown Reductionx9 or Greater Glyph of Cooldown Reductionx9

These are in my personal opinion all I want to spend my IP points on. Feel free to change these as you see fit.

Personally I think masteries make much more of a difference then runes do. I am not going to make an in-depth guide for this. For Trist i personally run a 21/0/9 build. I do not want to put the mastery page here when it is already at the top of the page. A 9/0/21 or 0/9/21 build could be beneficial but i have found the 21/0/9 build much more effective.

The reason I level up Rocket Jump first is for the slow it does. This really helps in killing enemies. Next up is explosive shot for the dot. The enemy is running away sou dot him and he dies. Rapid Fire I lvl up last because it is useful for pushing enemy towers. If you find yourself pushing towers then get some lvls in rapid fire.
Rocket jump is useful for jumping behind enemies then blasting them into your tower. This helps u pick up up games when u get your ult. Yup this is a really effective killing method. Using your ult to blast an enemy into tower range, they sometimes rage cuz its a whimp move or w/e. Its just a game get over it.

Recommended

Teleport is one of the spells I always take with Trist. It is useful for pushing an open lane or helping out teammates. It has many things it can do. This skill can make or break games in my opinion.
Flash is the other skill I always take with Trist. You can flash away from an enemy. You can use it to flash towards your opponent for an offensive use. This spell isa lifesaver and also a life taker.
Cleanse is a very viable pick. Cleanse+Flash=Clean Getaway. This is a great lifesaver.
Ghost is a viable pick. I don't usually take it because flash is way more useful in my opinion, but if you like ghost it is a good pick.

All the rest of the skills are not even useful for Trist in my opinion, but if you like them then go for it. Experimenting is what got me my build.

EVEN WITH THIS BUILD DO NOT THINK THAT YOU CAN 1v5 OR 1v2 THE OTHER TEAM. ALWAYS HAVE YOUR TEAMMATES THERE TO BACK YOU UP. YOU ARE A SQUISHY CHAMPION. 1v1 IS ABOUT ALL WE CAN DO ALONE! DO NOT GET TOO COCKY!

The starting item doesn't matter so much. I personally use a Doran's Blade to start but anything that increases armor or hp is viable as well.

Core Build

Shoes are necessary since Trist is very slow. Berserker's Greaves If there is no CC abilities on the other team. It gives some nice movement speed and some much needed atk speed. If there are CC (or many AP champs) then get Mercury Treads. This will help you shake off those stuns and make a clean getaway or continue attacking and getting the kill.
Infinity Edge is the best damage item in the game. It increases your crit strike chance, and crit damage as well. This is a core item for my Trist build.

Balanced Build Between Damage and Attack Speed

Last Whisper gives you the necessary armor penetration you will probably need. The enemy team by now realizes that you are in fact a threat. This item will help against the Thornmail's they will be building.
Phantom Dancer is the best attack speed/critical chance/bonus movement item in the game. This will greatly increas theammount of damage you do. The crits you get with this will more then make up for any damage item you coulda gotten.
The Bloodthirster is a great item for Trist. It gives some sweet lifesteal on top of amazing damage. Why not build this first? No one's stopping you, the passive makes it the top damage item in the game. Why dont I? Because the crit chance and crit damage on top of the physical damage that Infinity Edge gives makes this not as awesome in the begining but is still awesome for later on.
Cloak and DaggerGives you some nice attack speed and crit chance. On top of that it gives you some sweet tenacity.

Optional/Situational Items

First off the build above is only good if you have a good teammates. So if you are solo queuing then you are not going to do the build above unless you are confident that your teammates can handle protecting you.
The Black Cleaver is a great attack speed and damage item. The passive ability reduces the enemy's armor so it is a great item to get. {Credit goes to egotisticalnoob, tyvm for this item suggestion}
Thornmail Cheap and simply the best counter for AD heavy teams, if you see quite a lot of AD champs in opponent team, insta get it and enjoy how they suicide trying to take you down.
Guardian Angelthis item has saved my life many times. Since it refreshes your Q,W,E moves u can then rocket jump away or if they have low HP then finish them off. Take this item if you find yourself dying often, or u need a balance between armor and magic resist.
Banshee's Veil The best AP counter in the game. Probably. It blocks one of the enemy's spells every 45 seconds, that could mean life or death in some situations. For AP heavy teams this is definitely an item to get.
Force of Nature This is a great counter to AP teams. Has the best magic resist and gives you some pretty sweet health regen and movement speed. For AP heavy teams consider getting this item.

All of these items are just my suggestions for some situations. You cannot make a build for every situation. You should be able to change the build depending on what your enemies are making, to benefit you. These items above are recommended for some situations you will most likely run across. If you have any item suggestions please let me know!

This is my build, please let me know if you liked it or not. If you have any questions, i will answer to the best of my knowledge and abilities. If you have any suggestions to my build please let me know. I am open to suggestions. Thank you! ^-^