Stupid Mistakes to Avoid? Haven't played since 2016

After seeing groups of people on their phone walking through the part for a few weekends in a row I realized that Pokemon Go is far from the dead game I thought it was, and I reinstalled it and even got my old account back.

A lot has changed and I have a lot to learn, or course, but mostly I was hoping to find out what actions to avoid. For example, are there any resources I should absolutely hoard until later levels (only level 25 right now), or pokemon I should not evolve for some reason or another? Stuff like that.

I just don't want to kick myself in a few months/years for something that could have been avoided. Any advice on this is appreciated!

Edit: Thank you for all the advice, everyone! This got way bigger than I expected, but I really appreciate it and am taking notes!