Trying to Learn the Numbers

I have mentioned before that I most of the time shot my Nikon d50 on Auto. I have yet to shoot my new d90 on Manual. My class requires absolute manual setting and RAW format. No cropping. The reason I never really tried manual is that I have a problem with numbers (and I have 3 semesters of math for my chosen field of study!) and the umbers get confusing. You see, when you talk about Aperture you are talking about the opening in the lens that lets in light. Sounds simple, right? But right off the bat the numbers are backwards. The lowest number indicates the biggest opening and vice versa. Then you have shutter speed. I read that they are all based on one second. But you can dial the speed on your camera from as little as 3000/1 sec all the way up to a minute on most cameras. You can go faster and slower but the confusion comes in what speed I am supposed to shoot at what aperture. And then to make you more confused, you have to pick the ISO. At this point I know where the settings are and I know what high ISO and low ISO does but I DON’T UNDERSTAND what it IS. Thank goodness I have this college class and reading to do. I am determined to really understand this stuff!