How do I deal with hair that isn't really straight or wavy/curly?

So basically some of my hair curls, some doesn't so it looks horrible if I don't brush it but if I do it's just really frizzy. I had curly hair as a little kid but then it just kind of went away and now it's straight-ish but when I cut it (went from hip length to shoulder length) it curled a lot but now it's back to being long (waist length) and it does the frizzy shit again. It's honestly so annoying and I know that I could try a curly hair routine but I don't have any money to buy new products so I'm just stuck with a rat's nest of hair.

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Any advice on how to stop the frizz or honestly anything else that could help? Thanks in advance