From all that I've read and all that I've experienced, soil is the key to good gardening. And, regardless of what you have, you can always make it better. If "poor soil" means lots of clay and/or rocks, then remove the bigger rocks and improve the tilth of the soil by adding compost and some sand. If the soil is already too sandy, add lots of compost or topsoil so that the soil will hold more moisture.
I'd recommend tilling the soil on which you're going to place your raised beds. Remove bigger rocks and as much of the grass roots as you can. A tilled depth of 12" should be sufficient; but, deeper would even be better. You can kill the grass by covering it with plastic for several weeks, if you have the luxury of waiting that long. Otherwise, strip off the sod or pull out as many roots as possible.
I've used a little Mantis tiller for more than 20 years in my two raised beds. It really digs well and helps mix in all of the compost and sand that I add. The Mantis tiller also has a number of clever attachments that come in handy for other lawn chores, like edging and dethatching the lawn. They have a cute little animated site that shows all of the attachments at powertogrowityourself.com.
You can also rent a small tiller at most rental shops; you'll definitely get better soil - especially in a new garden - if you add compost, screened top soil, or sand (if necessary) and till it all in. Another great soil additive is pulverized, or finely chopped, leaves. I save mine in big plastic bags each fall and add them to my compost pile and to my garden. Finely chopped leaves make an excellent vegetable garden mulch, and they add a lot of organic material to the soil as they break down.
Good luck with your new garden. And, don't be discouraged if you have some little disappointments; we all do.
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