Start reading the nutritional facts on food packages. In the beginning it will make little sense. But as time goes by, you start understanding it a bit more and to notice patterns.
Eventually you start doing wiser choices. I've learned pretty quickly that the "healthy options" (e.g. low sugar cookies) are as bad for you than the regular ones.
Be sure to look at what they consider a serving size is when you do this. I've seen cases where you have something that is packaged as a single serving, but the nutritional facts say the serving size is half of that. I think this is just criminal. Like anyone would eat only half an instant ramen or whatever.
In the USA, this was supposed to be fixed in 2016 when the new nutrition facts label was introduced (the redesign that increased the font size of the calories).
By law, serving sizes must be based on the amount of food people typically consume, rather than how much they should consume.
Started really paying attention when my health imploded some years back. Would add that food content literacy tends to drive me to the outer edges of the grocery, and out of the middle where there's more junky, processed crap.