Most young people are doomed before they ever leave the supermarket.
If you leave the store with the wrong ingredients, or with a cart full of processed food, cans and boxes that will just rot away in the back of your pantry, you're not going to cook well! Good ingredients = good food. Now, that's not to say that you can't use canned elements with great success -- anyone who soaks their own garbanzo beans overnight instead of just buying canned either has way too much time on their hands, or is completely out of their gourd. This being said, canned gourds are a good idea as well (there is no excuse for NOT using canned pumpkin. Seriously, if you have the time to de-seed, scrape, roast, and puree a whole freaking pumpkin, get more hobbies. Or read a book. When's the last time you read a book?). We're going to use canned and boxed food here every once in a while, but it should be the exception, not the rule.
So, the supermarket -- and yes, a supermarket is just fine for almost all of your shopping needs. You don't need to have access to a brilliant butcher, a 4th generation fishmonger, an organic farmers' market. Of course, having access to all of these things is fantastic, and when you can get to the good stuff, for goodness' sake, do it!
This being said, 95% of the food shopping that the average person does takes place in a big-box supermarket. Hopefully, this post will help you do as well as you can with what's at hand.