You can try looking online for some take-apart guides, but its best to get an external case. What model of eMac are you getting? It got a little easier to upgrade the eMac's with the May 2003 revision, but not much better. Also, look out for raster shift on the first generation 700MHz eMac's, which causes 1/3 of the screen to shift up and the bottom half is black. It can be fixed by purchasing a new inverter cable. If you get one of the 1.25GHz models, those can be affected by blown capacitors. However, i've had my 1.25GHz eMac for almost 4 years and never been affected with anything. Good luck with it, mine has been superb and the CRT used is one of Apple's best screens ever.
RAM is extremely simple to upgrade. Just unscrew the bottom hatch and you're there. It's about the only thing that easy to upgrade.