Tristana is an AD carry which involves a very high range and amount of damage in late game,thanks to her passive which gives her a bigger range after leveling up.

TAKE INFINITY EDGE AFTER BOOTS!!!!
Tristana deals 0 damage if she has no damge items,so i strongly recomend buying Infinity Edge first,the a Phantom Dancer for the bonus attack and movement speed,taking Phantom Dancer frst is a very common mistake,but it can be repaired.
The Bloodthirster will give you an advantage on hunting the players,and restoring your hp fast,because Tristana is very squish,but fortunatelly she has a great escape mechanism that will make her a very annoying player.
Madred's Bloodrazor is a freat item for Tristana,making her resistant in fights,giving her bonus attack damage,attack speed and giving her another passive which involves damage.
The Black Cleaver is a very good item such as Madred because the bonus attack speed,attack damage and it also gives you the third passive which will deal damage along with the Madred's Bloodrazor.

Try using a W-E-R combo,if using smartcast this combo may be very easy.
Get with the W in the back of your opponent.Imediatelly use E,and then try to knock them into the wall with your ultimate,the basic attacks will do the rest along with your Rapid Fire.