There are lots of seeds just sitting in your garden waiting for you to collect them.  Peas and beans are easy, just start to save them after they are too tough to eat.  For tomatoes if you have time to soak them in water a few days to ferment off the goo, it is better, but you can also just put the seeds on a labelled paper towel to dry.  Flower seeds are everywhere.  Open a flower after it is done, and you will probably find them!!

Here is a link to a free download that is a book called "How to Start a Vegetable Garden" from the Farmer's Almanac
http://enews.almanac.com/q/5zdUFhXpvJU6oP619FUCMGiMDPuC-VbDQFnnaRaL_2ZASqkQZQLFztdXP


Here are some links to some great seed saving info.

Urban Tomato  https://urbantomato.ca/seed-saving-simple-starter-tips/
Seeds of Diversity   http://www.seeds.ca/diversity/seedsaving
Toronto Seed Library    http://www.torontoseedlibrary.org/
Seeds Richmond Grows (USA)      http://www.richmondgrowsseeds.org/seed-saving.html
Seed Savers Exchange (USA)   http://www.seedsavers.org/learn

Left Picture is Morning Glory Flowers
Right is Morning Glory seed pods and seeds

Below left is Hollyhock Flowers
Center and right are hollyhock seed pods and seeds
Picture
   Calendula - in flower at the top and bottom.
  On the right is a flower with the petals dying off
  On the left is a seed head with immature seeds
  Center has dried seeds ready to pick

Milk weed seeds need to go thru a winter in order to sprout.  It also helps if you rub them with sand paper.
Marigold Flowers above, and seeds from inside spent flowers above right
Black Eyed Susan Flowers above and seeds and chaff from spent flower heads above right
Radish above and seed pod and seeds above left
Purple Cone Flower above and seeds, chaff and spent flower head above right
Tomato Seeds - Scrape seeds and goo into a glass jar and add some water.  Cover and let it sit and ferment a for few days.  Then wash seeds in a sieve and dry on wax paper.  Easy!
Blanket Flowers and Seed Heads
Blanket Flower Seeds
Kale Leaf, Seed Pods and Seeds