It's really frustrating when the lights behind your gauges burn out, because you can't tell how fast you're going or how much gas you have left! Luckily they are easy to replace.
My car is a 2004 Impreza sedan. This procedure should be identical for all 2000 - 2007 models. Other Subarus will be similar but not identical.
Step 1: Remove the "Combination Meter Assembly" (aka instrument panel, or gauges)
The repair manual calls this the "combination meter assembly". Normal people call it "gauges", "instrument panel", or "instrument cluster".
Start by moving the steering wheel as far down as possible, to give yourself space to work.
Take off the black trim piece around the instrument cluster by removing 2 screws in the top of the piece. Sorry my picture is cut off. See step 2 for a better view.
There are also 2 snaps at the bottom of the trim piece, pull it directly towards you firmly until they "click" out. To get the piece to fit past the steering wheel as you remove it, try tilting toward you.