Tyler's 3 Gate Robo is indeed the best possible build to learn as your first build. Its safe and good against everything and you'll learn a lot while doing it from your losses/wins.
Regarding 2rax marines, you should move out with your initial stalkers immediately and try to damage/kill as many marines as you can before terran gets too many. If you stay in your base you will still die even if you have 5-6 stalkers to mass marines+scvs.
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