That info about your iMac model helps a lot.
It's pretty easy to get inside your iMac (Early 2008) model.
Remove the small panel on the bottom edge, where the RAM is installed
The front glass needs to be pulled off. Apple uses suction cups to grab the glass. It's held in by magnets. Then, you'll see 10 or 12 torx screws to remove around the edge of the LCD panel. Remove those, then you can pull the complete front bexel off.
8 more torx screws holding the LCD panel, then you can remove that panel. Careful of the various wires that you have to disconnect during the process. Some of the connectors are behind the LCD panel.
Once those parts are out of the way, you'll see the DVD burner, held in by 4 more screws.
Easy!
You do need to be really careful of the glass, and don't raise any dust while you have that apart - any dust and lint will be drawn like a magnet to the inside surfaces of the display. Also, don't touch any visible part of the display with your bare hands. Fingerprints inside the display get really annoying.