how do I replace a water pump on a 1990 pontaic firebird

Which engine do you have? Here's a video for a 3800 --- http://www.youtube.com/watch?v=DYFz-6_GJ_g

Tenspeed, google is helpful. They didn't put the 3800 in Firebirds until 95, excluding the 89 Turbo Trans Am. For 1990, your options were a 191 MPFI V6, 305 TBI or TPI, or 350 TPI. Since he just says Firebird, we'll assume he means the 191 (3.1L). Unfortunately, there aren't really any write ups on the web regarding that motor, so OP's best bet would be getting a Haynes or Chilton manual from th eparts store or finding a Factory Service Manual.

Samuel is right about the engine choices for a 1990 (assuming no previous owner did an engine swap on this 20+ year old car). All are pretty staight forward replacements if you have the knowledge and tools. Parts are readily available.