The easiest and cheapest way to improve your soil is to add organic matter to it each season. Grass clippings, shredded leaves etc added each year will help a lot. How is the soil tilth? Is it easily dug? If not, you could start by digging in some store-bought compost this spring. One other thing...don't plant the mint in the plot, rather grow it in a pot, or your plot will soon be nothing but mint!

Organic matter doesn't really make for "nutrient soil". It makes for better soil structure, and adds minimal nutrients. But no question that organic matter (esp compost!) is the best soil amendment for garden prep. Uncomposted organics are OK, but need some time to degrade before planting. The amount necessary? Depends on how bad your soil is. If your soil is readily friable with good tilth and drains well, maybe 10% will do it. If clayey and crusty, you could go up to 50%. Top soil is just junk. Don't buy top soil. It's largely just mineral dirt, scraped off the top of some random dirt pile. You have loads of that already.

With regard to nutrients, find out what you need before you start nutrifying. Soil tests are inexpensive, and can usually be done by your local extension service. Since we don't know where you are, we can't point you to it. They will test for major nutrients, and advise you how to fix if there are problems. Of course, once you plant, you largely can't go wrong by adding some general purpose fertilizer.