Hi Birdhouse Lady! Raised beds are the way to go!
Most plants don't need soil any deeper than 12" or so, but tomatoes are deep drinkers, so I would go a little deeper than that, probably about 14". After that, it's a matter of preference. Alot of people make them deeper so that they are easier to reach (higher) and you don't have to bend over as much. My own raised beds are about 22" deep.
Be sure not to make your bed too wide. Make sure you can reach the centre from each side of the bed, otherwise you will always be reaching to get to the middle. Also, if you are making more than one raised bed, make sure the paths between are wide enough. It should be wide enough to get your wheelbarrow through at least (30" is a nice width - depending on your wheelbarrow). Often people make the paths too narrow, and by the time the plants have grown in, there is no place to walk!
Growing in raised beds is much better for your soil as well. You are only adding nutrients to the garden bed, instead of the paths as well. You don't walk all over the garden, just the paths, so the garden soil doesn't get packed down. They also have better drainage, and warm up faster in the spring. You also can grow your plants closer together, getting more production for your space, and not letting as many weeds grow in between!
When you build your raised bed, make sure you fill it with nice healthy soil. Add lots of organic matter such as shredded leaves, compost, well rotted manure, kelp, etc. and your tomatoes and beans will love it!
Hope this helps! Enjoy it - I love raised beds!