Start by adapting things you already know how to make.
If you know how to make grilled cheese, how can you adapt it?
Well, you could use a different cheese. You could use a different bread. You could use something other than bread (like a bun, or some tortillas). You could add something in the middle, like bacon or onions or a slice of tomato. You could switch to leaving out the cheese altogether and just using bacon and onions. You could cook it open-face and top it with stuff rather than filling it. The list goes on.
The same thing can be applied to any recipe. If you know how to make spaghetti, experiment with different veggies. Try changing to a non-tomato-based sauce. Try different types of pasta. Try different grains altogether. If you take spaghetti, change the meat to chicken, then change the sauce to alfredo, then add carrots and broccoli, then put it over rice instead of pasta... well I bet you'd have a delicious meal, but each time you cook it would only be one step away from the previous meal.
Cookbooks are also good, but only good ones. I love to cook and love to read cookbooks, but I only own about a half dozen of them. A whole shelf full of a second-rate cookbooks is not going to help you. If you want to get a cookbook, make sure it's well-reviewed, reasonably popular and has things you actually want to cook in it. In my mind, a kitchen without Mark Bittman's
How To Cook Everything is incomplete. It is basically a modern Joy of Cooking. It has recipes for everything you could imagine, and it gives good instructions and background information. If you like Indian food, I recommend
660 Curries by Rhaghavan Iyer. It is the most foolproof cookbook I have ever used. The first time I cooked Indian food in my life it came out perfect. Everything I've made with the book since has come out perfect. The recipes are like magic. They
make you succeed. This is why I say to make sure a cookbook is well-reviewed. Recipes are not all the same. There can be good recipes and bad recipes. There can be good instructions and bad instructions. A cookbook can help or hinder. Get good cookbooks so they help rather than hinder.
I'm also a huge fan of Alton Brown, and watching Good Eats or reading his books can't possibly hurt. If you get Create (TV station) or the Food Network, you can almost certainly find more good people to watch.