I still suck at mobas, buying items fucks me up
For better or worse, ADCs all tend to build pretty similarly so once you the build semi-memorized, your set
Usually you'll want to build some combination of the following
Bloodthirster
Last Whisper
Phantom Dancer
Statik Shiv
Infinity Edge
Trinity Force
Blade of the Runed King
Berserker Boots
Guardian Angel
Randuin's Omen
Banshee's Veil
Mercurial Scimitar
of those, Bloodthirster, Last Whisper, Infinity Edge and Berserker Boot's are almost always standard, you usually want one AS item and one defensive item,
There are a few who build differently (Ezreal/Lucian usually wants to rush tri, Vayne rushes Blade) but for the most part thats it
Almost always build AD first,
though.
Caitlyn/Ezreal are the safest ADC's in the game, Caitlyn is pretty much always a safe pick (range, escape, no real downsides, ult is sketchy though), and Ezreal has a built in blink, and you can farm with his Q if your behind. Sivir is pretty cheap and, prior to the current patch (skimmed what it did to her, prolly should read it again), was top tier, Jinx/Lucian/Draven are the other top tier ones, but all 3 are remarkably expensive, and draven can be hard to play correctly.