All of the advice given here is really great. I would start with Duncan's recommendation, but refine it a little bit for you and have you start here:
RichardsPens.com • Pens That Write Right!
If you read bullet points 1 and 4 you will have a pretty good idea of how nibs and feeds work together.
In addition to all the links above take a look under the Resources tab at:
Edison Pen Co
One suggestion that I found worthwhile starting out: Buy a Churchill or El Grande kit. Before you even start building the kit take a good hard look at the grip section, feed and nib. Install the converter and fill it with some ink or colored water. You now have a fountain pen.
Wiggle--DON'T TWIST-- the nib and feed out of the section to get an idea of how this thing works. It all seems pretty simple, but historically it took a long time to get it right.
For what it's worth
Bill