How and when do I recruit Tali?
You will be able to recruit Tali on the Priority: Rannoch mission. Which you can't do until after:
The citadel attack by Cerberus.
It will be quite a while before you can recruit Tali if you just got Garrus.
Also, take note that you may not be able to recruit her:
If you do side with the Geth on Rannoch, one of the possible outcomes she'll commit suicide. If she died in mass effect 2 during the suicide mission, and you imported your save, then she will not appear in mass effect 3.