1) Masteries: it's a matter of playstyle really (at first levels) but you should focus on whichever tree gives you the most stats that you want, rather than split it around multiple trees. For 
Tristana, I'd probably focus first on the Ferocity tree.
2) Runes: Only worry about runes once you can buy tier 3 runes.
3) I believe it's something like Infinity Edge, berzerker's greaves, Rapid Firecannon, Bloodthirster, armor penetration (mortal reminder or lord dominik's regards, I believe most people go lord dominik's on marksmen), survivability item ( Guardian Angel, Banshee's Veil, Mercurial Scimitar) or more damage depending how confident you are in your positioning vs enemy team's engage.
Tristana, I'd probably focus first on the Ferocity tree.2) Runes: Only worry about runes once you can buy tier 3 runes.
3) I believe it's something like
Infinity Edge, berzerker's greaves, Rapid Firecannon, Bloodthirster, armor penetration (mortal reminder or lord dominik's regards, I believe most people go lord dominik's on marksmen), survivability item ( Guardian Angel, Banshee's Veil, Mercurial Scimitar) or more damage depending how confident you are in your positioning vs enemy team's engage.
you shouldn't need to bother with runes till you are lvl 20 as tier 1 or tier 2 runes are pretty worthless.
for masteries decide which stats are useful for you and which keystone 9at lvl 18 you can get one). Warlord's Bloodlust
is a good keystone and makes you go into the ferocity tree. most common tree for adc is 18/12/0
for items the thing False said is pretty standard. you also have the Statikk Shiv+ Rapid Firecannon ( Infinity Edge and beserker's greaves are core in it too) leaves you without a survivability option, lifesteal or penetration (mostly dropping the pen as they arean't really strong in the combo because shiv and cannon do magical damage and they are useless when no one threatening builds armour).
for masteries decide which stats are useful for you and which keystone 9at lvl 18 you can get one).
Warlord's Bloodlust
is a good keystone and makes you go into the ferocity tree. most common tree for adc is 18/12/0for items the thing False said is pretty standard. you also have the
Statikk Shiv+ Rapid Firecannon ( Infinity Edge and beserker's greaves are core in it too) leaves you without a survivability option, lifesteal or penetration (mostly dropping the pen as they arean't really strong in the combo because shiv and cannon do magical damage and they are useless when no one threatening builds armour).
Just some random guy from smitefire that visits sometimes.
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I'm still low level (5) and would like to improve my Tristana play.
As I'm levelling up, in what order should I spend my Masteries? Same thing for Runes.
Is there a recommended item build I should follow?
Thanks!