I want to make a garden where a chemically treated lawn currently is. How do I prepare the soil for planting?
It's great that you want to grow your own fresh vegetables!
Since the subject of your question is "Tomato Plants," let's start there. There's nothing fancy about the soil that tomatoes will grow in -- just make sure the grass gets killed completely and then till the remaining soil with a lot of organic matter (compost is best). Add some fertilizer, but what kind and how much depends on what kind of tomatoes you'll be growing. You can start with this article and then search our website for different types of tomatoes and other vegetables: https://www.gardeningknowhow.com/edible/vegetables/tomato/tips-for-growing-tomatoes.htm
(there are also a lot of helpful links in this article)