You can always start small with a garden, too! That way, you'll have more success. Often, people go dig up a huge plot and then it's too much for them to maintain.
You would be amazed at how much food you can get out of even a couple of large patio tubs!
If you want to dig up part of your yard, then I would suggest starting with just a few square feet and making it larger every year. That way, you won't skimp on soil preparation and weeding.
I would concentrate on growing things that you will get the most out of. I've grown most things you can in a garden, but here are some examples. I always like to grow an assortment of greens, because they're good for me, I like them a lot, they don't travel or store all that well, so they are extra fantastic when fresh out of the garden. We also have raspberries, because they also don't travel or ship very well, are expensive in the store and my husband loves them. Fresh, vine ripened tomatoes are fantastic, still warm from the sun. Yummy! Summer squash is easy to grow and I really love it. Winter squash stores well, after harvest. You might choose things that you like to eat, are easy to grow, don't take much growing space, store well after harvest, have a large harvest, are expensive in the store or are particularly nutritious.
1. Start small.
2. Do a thorough job of soil preparation. This is a more involved topic, depending on what your soil is like. It takes the most work, the first year. See above.
3. Choose things to grow that you really like to eat and it will make your gardening a lot more rewarding.
4. Keep it weeded and water as needed. Mulching can help keep weeds down.
5. Start a compost pile, to feed your soil and improve it's texture next year.
Good luck with your gardening! It's never too late in the year to start, even if you just get started on soil preparation for next year's garden. There are also fall and winter crops, depending on where you live.