A lot of people who are new to the software try just using the Chroma Key filter alone, which is usually a mistake, unless your green screen is really perfect. If the lighting it so/so or the shot isn;t the best, try using the methods I describe in this Composite Lab Tutorial
(same thing basically as VLab) and in my green screening series here
. They both talk about essentially the same thing- one in the form of text and the other, video demonstration.
I've gotten a lot of good comments on these videos- so the methods work pretty nicely that I talk about in those tutorials!