...if you have CS 2 you can use the live paint feature, a paint bucket with a star symbol in the tools palette...
...or you can use the merge command in the pathfinder palette and then have a duplicate set of lines above it to add a stoke to the lines...
...or you could simply place a soilid area underneath your existing lines that has a fill applied to it...
...in most cases the pathfinder palette is your best friend along with the live paint feature in CS2, very handy!!!
have fun!
andrew