Welcome to my Jax guide, as the name suggests, im a Jax main, im currently gold, but im an old school plat player
I've been playing Jax for over 7 years now, and have experience with Jax in every role and various situations

So for masteries you have some choices, fervor of battle is always a solid pick going 18-12, it gives you a good chunk of damamge while you autoing, and gives your spells ad as well, for most situations i prefer TLD, because of the burst it provides with third strike

So for starting items it all depends on what the lane is and who you are vs. Most of the time you will start Dorans Blade as your go to starting item. When vs strong AD champs like Darius, Xin or ADC top laners like Vayne, always go cloth x4
For more tankiness or more aggro mix damamge characters like Akali or Kennen start Dorans Shield
If its someone sustain heavy like Vlad or Maokai, start corrupting potion

Dorans Blade is usually the safest start for most matchups if you are not fighting a direct counter pick, it gives you solid damamge early along with some susatin and just enough health to win trades