Do a search for seed saving. Tomatoes need that extra soaking step to get the outer coating off but other seeds are saved the normal way of scooping out the seeds and laying them out to dry. Some, like cucumbers and okra need to grow large and way past eating stage for the seeds to mature. Lettuces and cole crops will bolt sending up flower stalks and eventually seeds will be produced and you can gather them by putting a sack over the stems and shaking the seeds into the sack. With flowers, clip off the dead blooms, put them in a labeled paper sack to continue drying. I usually have several little saucers on the kitchen counter of veg seeds drying naturally. Of course, labeled with some masking tape to identify which is which. Then they go into a small envelope with all the information on it. Some people put them into small containers like pill bottles but if they are completely dry, they may rot whereas an envelope can still breathe. After a month or so they go into baggies so the bugs won't get them. Many people store them in the freezer for longer storage prep but I have a continuous supply (knock on wood) of most things so they're kept in a box. That said, some bugs did hatch out in a bag of squash seeds so I put it in the freezer for a year but when I went to use it and left it out in a new baggie, more bugs hatched out and were happy munching away.
If you look on seed packages, sometimes they will say if the seed is a hybrid or an heirloom but sometimes you're left to guess. Heirlooms are seeds that have not been modified or tampered with for many years so the seeds will produce the same variety year after year. Hybrids have had various other varieties mingled with them or chemically changed, GMO, etc. so any seeds that were saved may be sterile and won't produce anything or the production may not be the same as the parent. Hybrids will give a larger production and it'll be more uniform (pretty) so it's your choice.