My hair is half-wavy, half straight!
The hair on the top of my head has very tight waves that are difficult to straighten, and the rest of my hair from about four inches down is straight as an arrow. It used to be completely straight, I don't know what happened. I have been using the same hair products for a very long time, but I'm trying to make the rest of my hair wavy (scrunching with mousse, sea salt texture spray, etc.) so I don't look like I just got out of bed, but it doesn't last very long unless I curl it with hot rollers and douse my hair in hairspray.
I used to have the same problem in junior high school. What I recommend you to do is divide your hair in sections like at least 4 sections and braid it. Or if you know how to cornrow then do 2 braids. Make sure to add product like mousse or gel and your hair will be curlier the next day with some hold, which it’ll make it last longer. Good luck!