Hi John,
The easiest seeds to start with will be annual self pollinating crops that do well in your climate. Peas and lettuce come to mind. These crops mostly self pollinate before insect pollinators get to them, so the varieties tend to come true to seed. With peas, as William said, they are the seed so let them mature on the vine and then harvest.
With lettuce, look at your 'days to maturity' for an idea of how early they will bolt. Romaines tend to be late bolting, so plant early and expect a late harvest. Other lettuces are earlier. Again there are many OP's available and will tend to come true, rather than crossing with others. I have grown several varieties fairly close to one another and did not find any crossed seed at distances of 2 to 10 ft. (If they do cross, all is not lost, it will still be lettuce and may be interesting!)
With self pollinators, you are not limited to one variety in a small space, so you can grow several kinds with fair assurance they will not cross pollinate.
Grains such as wheat and barley are in this category too, which self pollinate or wind pollinate, so that some effort or close interplanting of varieties is required to cross them, otherwise if planted in blocks of the same var, they will self and your seed will be true.
Another very easy seed crop is green onions (aka spring onions I believe). They are biennial, but it is not difficult to find varieties hardy enough to overwinter. I have one called Hardy Evergreen, for example, which endures our hard winters and flowers in early spring. So if your goal is to produce useful vegetable seeds, planting a patch of green onions to leave and overwinter is an easy step toward next year's crop. Same goes for leeks. Many varieties can be overwintered without any special protection, and in their second year they produce seed. They will not cross with green onions. Bulb onions are more complicated, you would need to harvest and store them, then replant the following year to make seed, generally.
Brassicas can be tricky because of their familial relationships and outcrossing habit. The asian greens which are annuals, you have to stagger your planting times so that you don't have two flowering at the same time that will cross. Yu Choy, Mizuna, Michihili or Napa cabbage, are among the many B. rapa varieties that will certainly be crossed by insects if they flower together. Turnips are in the same genus, and others.
Broccoli, cauliflower and some kales are B. oleracea. So you can choose only one from that group at a time, but you can make a seed crop at the same time as any of the B. rapa. Some types such as cabbage are biennial, you would need a good patch of them with protection I guess, to overwinter. (I haven't done cabbage).
Again, it depends on the crops you had in mind. If you have any specific questions, ask and someone can probably help.