Details for how to recruit Tali’Zorah nar Rayya.
You’ll automatically recruit Tali near the end of the Expose Saren phase of your first visit to the Citadel. After meeting with the Council for the first time, you’ll have to either recruit Garrus and/or Wrex, then head to Chora’s Den and defeat Fist. You’ll then be on a 4:00 counter to fight your way to the rendezvous Fist set up and defeat Saren’s assassins, rescuing Tali in the process.
After that you’ll take Tali to Udina and Anderson, Tali will reveal the evidence she has against Saren, and you’ll be compelled to take Tali along with you for the rest of the game.
You’ll unlock the Quarian Ally achievement for completing five missions and/or assignment with Tali in your active party.
ME1: Complete 5 missions with the quarian squad member