Answer: First of all, if there are lots of tree roots....are there lots of trees? Remember that for best production, a vegetable garden should be in full sun; that is, it should get at least 6 hours of full sun, preferably more.
If the tree roots are from living trees, and not stumps, I would not disturb them by rototilling. In fact, be aware that covering a large percentage of a tree's roots with extra soil can weaken and even kill it. (That's why it's not a good idea to spread extra soil under a tree to cover surface roots.)
That said, I would leave the ground as is, build your raised beds, and fill them with topsoil or a topsoil/compost mix. This will smother any grass underneath, so don't worry about killing it first. I would not cover the grass with plastic, as this might inhibit drainage.
You might want to have two types of beds. Shallow beds (6-10" deep) can be used for small, shallow-rooted crops like lettuce, spinach, beets, radishes, etc. If you want to grow larger plants like tomatoes, or plants with elongated taproots like carrots, you'll need deeper beds (12+ inches deep).
Hope this helps get you started!
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