I usually will start by adding compost is a 50/50 ratio. Half soil/half compost. Then every time I turn the soil to add plants, remove dead plants, I add more compost at 10-20% compost depending on the condition of the soil Compost is made by taking plant material - and allowing it to decompose, usually heating the compost pile enough to kill weed seeds and insect larvae. It should be nice a black and have a earthy smell.
Double digging is also recommended, but I don't double dig. Its a great way to burn you out on preparing your soil. Double digging requires you to remove the top 8-10 inches of soil - set it aside, the dig compost into the subsoil. Then you add compost to the pile of soil from the upper levels and return the soil to the hole. This requires a lot of work and you can only get a small section done. It's a great way to improve your soil, but I have 2.5 acres. I would never get anything planted because I would be double digging all the time.
Here are some pictures of compost.