- You identify the plants you eat a lot of and are expensive and grow those. Eat a lot of russet potatoes? Probably not a good idea to grow them, because they're so dang cheap. Like chives? Well, they are really easy to grow, and cut herbs are expensive in the store, so go ahead. Raspberries are also super easy, and pretty expensive. My general rule is: grow leafy vegetables, herbs, and berries. Buy root vegetables and squashes, plus anything that doesn't get enough heat here. This is a of course a very generalized rule; there are superior and exceptional tasting varieties of every crop, and if you really love eating them, than it might be worthwhile.
- Find out what crops grow well and easily for you, in your climate. For me, beans and squash are tasty, yes, but we just don't get a big enough crop to make it worthwhile. However, I grow a lot of kale, swiss chard, and chives, because they just do fantastically here.
- Develop your soil's fertility cheaply or for free. Use composted kitchen scraps and lawn clippings, check out coffee stands and see if they'll give you coffee grounds, collect deciduous tree leaves during fall. All of these are free or nearly free sources of fertility that will make your garden really produce. Coffee grounds are particularly useful because if you layer it thickly enough it acts as a wonderful weed block.
- Don't get carried away and buy a pile of tools. Really, unless you have a big garden, all you need is a shovel, a trowel, and maybe a rake and garden fork. You do not need to buy rototillers or other fancy and expensive doodads.
- Realize that gardening is a skill, and may take some time to develop. Some people are fantastic their first year, but many have entire crops fail before they figure out what they need to do.
If you're looking at per sq ft yields because you've got a small garden like me, this table is a good stating place. This site is also very informational, and tracks a garden's costs and profits through the year.