Help me please

So I am 20 years old with a 15 month old and a 4 month old. I go grocery shopping or well at least I try to but as bad as this sounds I don’t really know how to grocery shop, I mean obviously I know how but I mean I but snack like things and just things that don’t last or make meals.. so I please need help on how to get things I’m actually gonna cook and make and use. What do you buy when you go grocery shopping and what do you make for lunch and dinner?

The reason I never “learned” how to grocery shop is because growing up all me and my brothers were ever fed was fast food for every meal.. I don’t want my kids growing up like that so I need to learn how to go shopping and make the most of it..