You don't need to buy expensive products.
Here's what I did.
Start with a cleanse. I actually used apple cider vinegar and bicarb of soda mixed with my shampoo. Lifted and cleansed my roots amazingly.
Use a sulfate free shampoo and conditioner. I'm using OGX Coconut Curls and Hask Coconut conditioner. Aveeno is also good.
When you condition, comb through the hair with a wide comb to form clumps.
I have thick long hair so I plaited it into 4 sections and clipped Them up and left it to deep condition for 30 mins. Probably not essential.
Rinse thoroughly.
On either soaking wet hair or squeeze some moisture out with a microfibre towel or tshirt (experiment with wet or drier hair) (normal towel can create friction or frizz).
Then add a Curl creme and run through hair with hands in a prayer 🙏 motion over sections of hair. I use Aussie Work That Curl. Not expensive.
Then get a gel, I use one from Noughty as it was on offer but I heard Umberto Giannini is good too.
This part is the most important part. Scrunch upwards for about 5 minutes with the gel. You can rotate your head while doing it, lean forward, sideways etc.
Then either leave to air dry or gently diffuse.
That will give you a basic idea of how your curls will look.
Another option is to Google Curly Hair Salon and find a curly hairdresser and go for a curly cut or wash and blowdry. There are now quite a few who will show you the whole method to achieve curls from wavy hair and teach you it.