Installation is fairly easy... You figure out what type of RAM your computer uses, how many freem RAM slots there are, and simply plug it into the RAM slot... This is LITERALLY a 5 minute upgrade, from the time you turn the first screw until the time you tighten the last screw on the case cover.
If you're in the US, buy from newegg.com, and the ram you should be looking for is 184-pin DDR SDRAM. According to these specs: Dell Dimension 3000 (Celeron 2.4 GHz, 256 MB RAM, 40 GB HDD) Specs. Desktops Specifications.
You already have 256mb installed. Now you can either, take out the current 256mb, and install 2x512mb for 1gb. Or get one 1gb stick, which will make it about 1.2gb of ram.