[montewilson (Monte)]

You'll find loads of free info on the net for all those questions.

A Dark is a shot with the same camera for the same time at preferably the same temperature (of the camera) but with the lens cap on. When you do it you will see lots of little "stars" This is just noise in the camera and when it is is combined with the real image and then subtracted these "stars" will dissapear. Have a look at the good pictures posted by some of the members and you will see they are not there.

A flat field is a shot of an evenly illuminated area (white wall e.g.) with the exact lens and camera combo as in the photo.
You should have a greyish image with the centre more illuminated than the outsides. If you apply this with the appropriate software you will see the sides of your shot brighten and the centre fade (as in the example in the reply) and it will be a more pleasing image.

Also go to 800 and try going down a stop to 5.6. If you don't get coma (little comets in the corner of the frame that point towards the centre) or it is minimal try one more stop. I am guessing for a cheap lens 5.6 would be the limit. If you are not sure of what I am talking about take a shot at f1.4 or the most open you have and you will see!

I can't recommend any free/basic software but someone else surely can. Go the software forum in IIS and ask there. They will certainly know.

When you have more confidence with some software you can stack multiple exposures and create some really great shots. Your equipment is capable of some good work, you just need to be patient and read up as much as you can here and in the wider web.