1. Read the EVN Bible. You don't have to memorize everything, but try and get a general idea of what you can and can't do with a plug-in.
2. Start small. Make a plug-in that changes the speed of a ship, or the price of an outfit. Better, start by downloading other people's plug-ins and seeing what they do and how they do it. Then start making your own stuff, like adding a new item to buy or variant ship, eventually systems with their own new planets and missions to fly there.
3. Do not try and start a total conversion (TC in the lingo of these boards) until you can't remember having ever not known how to manipulate each of the various resources.
4. Most important, accept that what you make (at least at first) won't necessarily be as good as what you find in the game, and it doesn't have to be. I've been making plug-ins for Escape Velocity since it first came out, and I still can't do what Matt Burch did, let alone ATMOS. Don't let it discourage you if you don't immediately become the next Martin Turner.
5. Come here for help, once you've gotten your feet wet. Some of the boards members can come up with insane solutions to do stuff that Nova can't normally do, but the rest of us can help with a plug-in that doesn't seem to work, or a system that doesn't show up, or a weapon that doesn't fire, or an outfit you can't buy...
------------------
To quote a pillar of American society: D'OH!