The original T-800 heard the call from Sarah to his friend Ginger, and besides Kyle was following Sarah around, but how did he (Reese) find where she was at these disco?
Reese knew precisely where she was because he'd been tailing her from her apartment. His goal was to surveil her until he could detect the Terminator, then (try to) kill it and get her away from it.
She rode off the kickstand and putted slowly down toward the cars. As she passed the dusty gray sedan, she looked in and saw that there was no one there. When she reached the apron, she paused, checking for cross traffic.
If she had looked back, she would have seen someone sitting up in the front seat, slipping his scarred hands on the wheel.
Note that Reese was expecting that with his knowledge of Terminators he'd be able to kill it and was taken aback when it kept coming, despite him loading an entire shotgun into it at point-blank range. He clearly wasn't anticipating that Skynet had sent a late model T-800 with all the trimmings.
He must have been a seven hundred, Reese thought to himself. He stood there for a heartbeat, the adrenaline still pounding through him, making everything seem slower, feeling the rush, as if he were riding on top of the wave.
Reese had fired in midair as he leaped from the back of one vinyl-covered booth to the next. He landed agilely and came up firing. He fired again— Die, fucker! — and stroked up another shell. The Remington kept exploding, one blast after another, brutally, savagely, unrelentingly, pushing Terminator backward, toward the plate-glass window.
As to why he didn't make contact with her earlier, that's explained shortly afterwards
“I had to wait till he moved on you before I zeroed him. I didn’t know what he looked like.” Sarah realized he was referring to the people who were injured or killed at Tech Noir.
And obviously she needs to see a Terminator or she'll just assume he's a crazy person and run away from him.