Great! The first thing you should probably do is to join the Railway 32 mailing list, and the r32usa mailing list if you plan to work on American railroad equipment. It's a good idea to post a message to the appropriate list saying what you intend to work on; this is to make sure that nobody else has already done it or is doing so -- you wouldn't want to spend forever drawing something that someone else has already produced.
The next thing you want to do is to acquire reference material. It's quite a bit of work to do this; you wouldn't want to do it wrong. Photographs of your intended subject, dimensioned drawings if you can get them. You may already have these; a good place to start is with what you know.
Then, draw away! It's probably a good idea to study how other R32 artists do things, but I generally start out by laying out a rough outline of what I'm going to draw, then work on adding detail.
You can use pretty much any pixel-based paint program, but I'd advise against Windows Paint if you can.
First, make sure you're requesting, not demanding. I do this for fun, not out of obligation, and I'm sure I speak for everyone else who does this in saying that.
Which is not to say I'm some grumpy curmudgeon who won't help you. I like doing this, after all, and I like my work being appreciated. I'm quite willing to help with any polite request.
If you can't find a drawing for something you want, there are two possible reasons: either I lack material to work from, or I haven't got round to it yet. If you want something done, it's a good idea to ask if I need any help getting scale drawings, photographs etc.
I use The Gimp on Linux to do most of my artwork these days. A lot of my older work was done in Corel Paint on Windows. I've also used Paint Shop Pro and Photoshop.
All of these have their good points and their drawbacks. I use The Gimp mostly because the system I prefer to use for day-to-day computing is Linux. That said, it's a very good graphics program, very easy to work with. My biggest beef is the lack of keyboardability of a lot of its functionality (oddly enough, Linux software is poor at keyboard shortcuts).