Answer:

Look for Open-Pollinated Plants

It is easiest to save seed from varieties of vegetables and plants that are open-pollinated.  These seedlings typically resemble the parent plants as long as they are not allowed to cross-pollinate with another variety of the same species.  In contrast, F1 hybrids are produced by crossing two specific varieties and planting the resulting seed.  Seed saved from hybrid plants will not produce progeny that resemble the parent plant and in many cases they produce inferior fruit or flowers.

Start with Self-Pollinated Plants

Self-pollinated varieties of plants are easier to save seed from than cross-pollinated types.  Self-pollinated plants do not require an external vector, like wind or an insect, for pollination and are more likely to produce seed that resembles parent plant. When an open-pollinated plant requires cross-pollination, you can still get true-to-type seed if you only grow one variety of that species in any given season.  These plants can also be isolated by distance, grown in containment tents made of netting, or grown at different times of the year to produce true-to-type seed.

The Best Plants to Start With

The easiest plants to save seed from are annuals that are open-pollinated and self-pollinated.   Beans, lettuce, peas, tomatoes, and peppers are great vegetables for beginning seed savers.  Flowers great for seed saving include marigold, zinnia, nasturtium, poppy, snapdragon, and sunflower.

Read more: DIY seed starting mix recipe