Besides the new seeds that I order, I also save unused seeds from the previous year and they generally grow very well for us. And, we save seeds from the heirloom vegetables that we grow. Depending on your climate, you may need to take measures to keep them viable, but I generally just put them into my expanding file/folder, sorted alphabetically. To test old seeds, you can fold a few up in wet paper towel and keep them moist for a few days or longer to see how many sprout. If a percentage of them begin to grow, you should be able to trust them for a garden planting.
Now... what do all of the the descriptions mean and how do we make the best choices? First of all, don't grow things you don't like :) If you are just starting a garden, choose things you and your family will want to eat. From the beginning,(1976) our family gardens were huge because we like everything, but maybe tomatoes, peppers, salad greens, green beans, and summer squash would be good beginnings.
Space requirements: Some things like radishes and leaf lettuce take up very little space. Others, like zucchini, will easily take over a 4-6 sq. ft. area. Cucumber and winter squash vines can run on forever......
Climate, of course will be a consideration in not only when you plant, but the time to maturity of each vegetable. Sweet potatoes, for example can require hot weather for 100 days or more and may not do well for us here in northern Ohio. Our greenhouse extends our season for about one month in the spring and one in the fall, if I plan my planting times carefully. Hot caps, or covered boxes can do the same.
And then, it's important to know how much produce you will get from one seed. My husband's favorite story is about hearing a girl in a farm market tell someone that she thought one seed would produce one zucchini, so he should buy a lot! Oh my! I bet he was surprised! Some vegetables like radishes, carrots, beets, turnips, lettuce, spinach, and cabbage produce one vegetable. Others like tomatoes, peppers, and green beans produce a plant that will give you many vegetables over a period of time.
Heirloom varieties are those that have been around for many years. Their seeds are "true" and they will produce the same plant and vegetable year after year. They also seem to have the best flavor and that's what we grow them for after all.
Hybrid types were developed after WWII, when people started buying produce from larger grocery stores, instead of neighborhood markets or growing their own. Commercial markets demanded produce in large amounts, all the same size, shape, and color. Plant breeders began cross-pollinating different varieties of the same vegetable - tomatoes for example - to produce a new variety that would be more productive, more disease resistant, or produce the whole crop at the same time for more efficient harvesting. All of those things are good and may be important to you.
Organic seeds may be either of the above, but that label means the crop producing the seeds for sale was grown without pesticides, herbicides, etc. What you do with them after that is up to you.
GMO seeds are relatively new and produced by altering genes within the seed itself to grow only under certain conditions. These are reserved for commercial growers and you need not worry about finding them in suppliers who are offering seeds for your backyard garden. Nor would you want them.
I always receive a large number of garden catalogues in the mail. These days, it's just as easy to search on line for suppliers. I order my favorites, read descriptions on what's new, and usually order from several different places. I would suggest that you start early. Take the time to study the offerings, but don't hesitate too long. Because of the events of last year, many more people started growing gardens and supplies quickly became difficult to find. Of course, the farm stores and other big box stores will have seeds and supplies in a month or two, but if you are looking for specific items, it's best to get started now.