A realistic demo derby game is something most people in the sport never thought would be possible. My motivation for modding is entirely driven by my passion for the sport, I don't think I'd have much interest in making a mod for a different topic. I knew it was possible and knew it wouldn't get done on it's own. I learned to mod mostly on my own, with a little help here and there from others. As others have pointed out, nobody will spoon feed you advice for modding because 95% of people give up right away and won't get far if they can't learn to find things on their own. I used to write out step by step advice to everyone who asked, now I just point them to the wiki... I enjoy it very much, I'm not a huge fan of modeling, but don't hate it either, and I like jbeam a lot and get a lot of satisfaction from seeing my progress.
I got into modding long after I first bought the game, it started with me making crappy automation mods with gradually increasing quality, editing them to e.g. have a horn was what got me into beam modding in general. I think the first real """project""" was increasing the torque and other things from the bus a little and now I'm making maps and engine swaps and sht.