I'll take a shot at these, since I'm up anyway. (2:20 am here, can't sleep)
1) You don't need a step for weight training, and in fact I can't really think why you'd want one for this purpose. Lunges, squats, etc can all be done without a step. I suppose you could do calf raises with a step, but any ol' stair in your house or yard (or even the curb!) would do just fine.
2) Incline treadmills work your quads (thighs), I suppose. But if you're a beginner, don't worry too much about isolating particular muscles. You'll get the most bang for your time & energy buck out of compound movements like squats, lunges, deadlifts, bench press, rows, and so forth.
If you haven't found it already, I strongly recommend you read Krista Scott Dixon's website,
www.stumptuous.com. Best info on the web for women who are just starting out, and she's not trying to sell anything. An hour or two on that site, and you'll have all the info you need to get you lifting and eating right. It's inspirational, too. (OK, enough raving.)
3) There are two problems with your gym teacher's formula method. One is that it requires that you know your one-rep max, and the only way to do that is to lift at maximal effort for one rep. It isn't a good idea to try this when you're a beginner, and still working on your form on various exercises. Second, it's also very imprecise, because the 50% "rule" is just a formula. Bodies are different, depending on the amount and type of muscle fibers ("fast twitch" or "slow twitch", in case you care). In fact, women tend to have better endurance than men, so a formula he gleaned out of some bodybuilding magazine in the 70s probably doesn't apply to women well at all.
A better way to find out how much weight you should be lifting for 10 reps on an exercise (or whatever) is to try lifting X weight for 10 reps! If you can do 10 reps easily at weight X, then try 10 reps at the next weight up. If you can only do 6, then move down a weight. Keep track in a little notebook, so you can remember for next time.
Form is key, though. Work on form, even at a lighter weight, and then add to it. Again, see Krista's site, where she lays out some basic exercises and the incorrect and correct form for each. She offers a lot of good basic routines, too.
Good luck, and three cheers for deciding to build weight training into your exercise plan.