I don't know tons about movie editing, so please try to be descriptive and simple! sorry for the inconvenience
Hi
You can find lots of tutorials on green screen work on the Internet 
here is one on how to setup green screen for a starter which should stop you having many of the problems associated with green screening - note this is a pro tutorial but the principles are the same whether pro or not.
John
wiseguy,
The tutorial John found is excellent for getting the best possible results but I'd suggest in addition a simple 'suck it and see' approach to get you started with MEP. You can even try it with a couple of still images.
Use timeline mode. Place your 'background' image/clip on track one and your subject directly below (higher number track). Select your subject object and go to the 'Effects' tab >> 'Video effects' >> Chroma key' and click on the 'Green' icon. That's it -- job done! sort of... :-)
You'll see that there are options for blue screen as well, and also black and white though they are probably better for graphics art design as most normal subjects will probably have some areas sufficiently dark or light to become transparent. Note also the the slider for 'Threshold' and 'Fading range'. these allow adjustment for how far (on both sides toward yellow or blue) from pure green will be transparent and whether the edges are sharp or soft. The image below is handy for testing all the chroma effects as it's clear what's happening:
'Alpha' is my favorite, but that's another story!
Regards,
Carl