Tristana is a very good Ranged DPS Carry. Tristana is near impossible to gank and a very good ganker on others.

Tristana mid is better. as it gives you a head start in levels and cs, also you can choose from 2 side lanes to gank. and do it fast so you dont lose towers.

With your insane range you can snipe some low hp champs inside the towers with no harm to yourself.

also your rocket shot cd is refreshed on kill or assist so diving a turret for a kill(if you know you can kill the champion) , you can easily just rocket jump str8 out.

You might be asking WTF? Health on Seals. YOUR A CARRY!.
getting dorians blade and having those health runes will make your more durable early game.

without these things tristana only has 4 bars. 4 hits from xin and your finished.
with this you would be like 6 giving you a little advantage early game.

Not difficult to understand, explosive shot gives easy wave take outs and lots of early cs.
its also a GREAT harassment tool, as champions walk by minions you kill they take damage.
and you can still shoot it on the other champions to do a DOT for maximum harassment.

I Get Rocket jump second for the simple reason. some junglers gank at just lvl 2 so getting rocket jump second gives you the anti gank tool you need.

You need a least 1 on Q For maximum dmg to champions if in a 1v1 situation.

Ult at all lvls for the fact it knocks people back further with each lvl. giving you more time to attack your enemy before they catch up to you.

I like ghostblade for my first offensive item as it gives you a ghost like speed boost and it gives you that armour pen you need for mid game team fights. its also good synergy with you Q skill.

Phantom dancer gives enough attack speed for the rest of the game and it has good synergy with ghostblade!

if 1v1 against any melee champion always use your ult as a kind of exhaust. if they are pushed away from you how do they do dmg?

Pros
Nearly impossible to gank
Insane range
In built ignite
can support friendly champions with your ult eg. ulting enemies away from your team. janna like ult.

Cons

Very Squishy
Ult can be gay if done wrong
not alot of lane control/cc