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
Wild Seed Project (USA) https://wildseedproject.net/what-we-do/
Here is a link for safe alternatives to Round Up https://www.consumernotice.org/environmental/pesticides/roundup/alternatives/
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
Wild Seed Project (USA) https://wildseedproject.net/what-we-do/
Here is a link for safe alternatives to Round Up https://www.consumernotice.org/environmental/pesticides/roundup/alternatives/
Black Eyed Susan Flowers above and seeds and chaff from spent flower heads above right
Purple Cone Flower above and seeds, chaff and spent flower head above right