This forum is a great place to learn- probably better than that teacher you had.

Such snobbery. I'll never be the next Ansel Adams either, but I daresay I'm at least a bit more than functional. Most of the mistakes you made in your shots can be corrected by just learning some techincal aspects, such as the ISO thing... composition is harder to understand, but elements of that can even be taught.
Stick with it. As long as you listen and learn from what you are being told, you will be fine.
BTW, there are TONS of posts about ISO, so search and read up. There was one that was posted just yesterday that explained the whole HI LO thing in pretty good detail, for example.
What tired said is absolutely correct, but another way to think of it is this...
High ISO = needs less light, faster shutter times, less color saturation, more noise
Low ISO = needs less light, slower shutter times, more color saturation, less noise
You dial ISO up or down as meets your needs, keeping in mind that the lower ISO is going to give you a better picture quality
assuming you can capture the image with a proper shutter speed. So, as tired said, generally speaking you want the lowest ISO you can use while still having the right shutter speed for the job.