Though Tali appears in the first game, she isn't courtable in Mass Effect 1 alongside the three partner options Kaiden Alenko, Liara T'Soni, and Ashley Williams, though players can still build a relationship with her. Tali can be wooed starting with Mass Effect 2 and in Mass Effect 3 as well, and many find that her story is worth sticking around for.
This article contains mild story spoilers for Mass Effect 2.
Tali'Zorah nar Rayya is an alien that first boards Shepard's ship after she accurately provides essential information on a certain Spectre. A quarian, Tali hides her face behind a mask and isn't used to being treated with respect, if Shepard chooses to do so. In Mass Effect 1, Shepard can discover that she's not used to such a quiet vessel, which usually means something's wrong on a quarian ship. As soon as the first game, Tali will faithfully fight by Shepard's side and provides the team with important intel along the way. Though Tali may leave Shepard's command at the end of Mass Effect 1, she returns in 2, this time, as a love interest only for male Commander Shepard.
Some might find it unfortunate that Tali can't be romanced in Mass Effect 1, but if they continue on with the trilogy, her story arc is rewarding. Tali returns in Mass Effect 2 having completed her pilgrimage, meaning she's a young adult who's proven her worth aboard the Normandy. Tali is the first former squad member Shepard will be asked to recruit in a dossier. During this mission, make sure to save Kal'Reegar by telling him to stand down, which increases Tali's loyalty levels.
Not too long afterward, crewmates will start telling Shepard that he's "all she talks about," hinting that she might have feelings for him. She will eventually confirm this herself, during which time Commander Shepard can reciprocate her romantic interest. Following this, players simply need to complete her story up until the final mission by continuing to speak with her.
As most players know, the Mass Effect trilogy rolls over information from each game, including who was romanced in Mass Effect 1 or 2 to Mass Effect 3. However, in order to do this, players will need to ensure that she survives the suicide mission. This is done by securing Tali's loyalty, which happens during the Treason: Tali mission. During this, do not present evidence of her father's crimes either with "I don't need evidence" or "We have no evidence," keeping her loyalty with Shepard.
After completing Tali's loyalty mission, she and Legion will get into a fight. Shepard can quell both of them by saying "You're both right" or "Knock this crap off," but they both require high persuasion checks. If Shepard can't make the check, make sure to say "Legion, stop transmitting." This means antagonizing Legion, but the alternative, "Back off, Tali," will immediately end her romance.
Mass Effect: Legendary Edition is available on PC, PS4, and Xbox One.