Can I substitute fresh tomatoes for canned?

Recipes that call for canned tomatoes usually do so because these are picked and preserved when at peak ripeness, as well as saving you the trouble of peeling. For whole or chopped canned tomatoes, you can substitute fresh tomatoes (get the ripest ones you can) that you'll have to peel yourself. For other canned tomato products (like passata or paste), you will need to process the fresh tomatoes to get the same result.

Note that if the recipe calls for canned tomatoes and a pinch of sugar, you might want to omit or dial back the sugar. Recipes often include sugar to counteract the higher acidity of some canned tomatoes.

Yep, fresh tomatoes will work fine. You'll likely need to cook them longer than you would canned tomatoes, and you may need to use more. If you like, you can skin them first: slice a wide, shallow "X" on the bottom of each one and drop them into a pot of water at a rolling boil. After a minute or two, the skins will have peeled back at the X and can be peeled off. (Discard the water.) Then chop them. (There's no need to finely chop them: they'll break down during the long cooking.)