TCCL Seed Library divides seeds into three levels of seed saving diffciulty: Easy, Medium, and Advanced.
Easy seeds are self-pollinating and require no special isolation techniques. That is, they pollinate themselves with minimal crossing from wind or insects. Easy seeds are perfect for beginning seed savers.
Medium seeds may require some isolation techniques or be biennial. Medium seeds are fun for intermediate seed savers.
Advanced seeds require careful isolation through time, distance, and mechanical means. Flowers may need to be kept closed and hand-pollinated, or the plants may require isolation distances much larger than the average home garden.
These plants generally self-pollinate, so you don't have to worry about out-crossing, and make seeds in one season. Just let them ripen fully and dry completely, label and store.
*Tomato and pepper seeds have a thin coating that inhibits germination. Fermenting that coating off will make the seeds more successful.