1ACV01: Space Pilot 3000
Here is the very first outfit Leela ever wore: her Fate Assignment Officer uniform. It's mostly like the usual clothes she wears : black jacket, with her little officer insigna, tight black pants, and her classical gray boots.
Here's a pic by Jon Dixon.
Another pic: Leela in the classical outfit she wears in every episode. White
tank top, tight black pants, gray boots and, of course, her Wrist-Thingy (TM).
We first see it at the end of Space Pilot 3000, when she takes off her
officer jacket. She was just wearing the tank top under the jacket. (Picture taken from Futurama Scan Art Central. Scan Art by Paul A. Metcalfe.)
Below is a picture by Jon Dixon.