To get that greenscreen "keyable", you need to make it so that it's all as much of a single colour as possible.
And how do you do that? By making sure that it's VERY flat indeed. If you look at your shots, the greenscreen is all scrunched and wrinked, and this means that there are multiple shades of green on the screen rather than a single, flat colour. When attatching it to the wall or whatever, pull it really taught, and stick it down with LOTS of duct-tape or similar. That'll make sure that there aren't any creases which would make keying impossible.
You could also do with some extra lighting on the screen, behind your actor. If you can't do this with actuall lights (cheap ones from a hardware store will do) - then you can make some 'reflectors' for next to nothing that will help you make the most of your natural light sources. To do this, get a big, thick cardboard box, and cut it so that you have a large, flat section of cardboard. The larger, the better. Then, take some aluminium foil, and open it up. There are two sides to the foil - one's REALLY shiney, and the other is sorta dull. You want to stick the aluminium foil down so that the DULL
side is upwards (so that you can see the dull side and not the shiney one). Try to stick the foil down so that there are no crinkles, creases or wrinkles. Once it's stuck in place, you're done.
Now, with those reflectors placed (out of shot) angled towards the screen, between the actor and the screen itself, you'll be bouncing the natural light away from the actor, and onto the screen instead. This should make the thing easier to key. Make sure you try and get a 'flat' lighting - in that you don't want parts that are lit better than others (you can play around with this by moving the reflectors further or nearer to the screen and angling them differently).
Here's a [very hurried and pretty rubbish!] picture to help you understand what I mean about their positioning :
You also need to sort out your sound. Get a set of headphones, and plug those into the camera's headphone socket, so you're able to tell how the sound levels are (in this case, they were far too quiet to be heard). You might also want to invest in a cheap mic - you can get them from audio shops for very little. Then, get a broom handle (they're only about £1), an audio extension cable, some duct tape and someone to hold the damn thing - and you've got a crude boom mic for very little money indeed! Obviously it won't be as good as a proper one - but it should be a massive improvement over what you've got at the moment.
I think it would also help you to get a better idea of what it is you want to shoot before you do - scripting and storyboarding are very important here. Just take a couple of sheets of paper, and sketch out how you want each scene to look, shot by shot. It doesn't have to be incredibly detailed - but just as long as it's enough to help you clear up your own mental picture of how you want the film to look. You might also want to let your actors learn their lines a couple of days/weeks ahead of time, and then do some 'dressed rehersals' or dry runs - going through the exact thing as if you were filming it, only without videoing anything.
I hope this helps