I have a strong belief that any hair can hold curl. A lot of clients swear that theirs is the hair that will never hold curl, and it is my true joy to prove them wrong.
All you need is products. Possibly handfuls upon handfuls, but that is all you need.
My very favorite foundation is mousse. I talk about mousse all the time, but I don’t know that I ever get much more specific than saying that it is great for volume. The truth is, mousse is the world’s greatest foundation product. Note: sometimes my clients are shocked to see that I might use two or three handfuls of mousse in their hair. That is when I don’t have time for nonsense (or misbehaving hair).
Here is what you’re going to do. Shower (or just shampoo/condition your hair). Towel dry. Put in several handfuls of mousse. Every inch of your hair will be moussed, root to tip. The key is to take the silky/softness out. Then you are going to comb through. Then you’re going to blow dry. Then you are going to curl your hair and possibly set it in pins (while it cools). And it is going to hold.
All day. Can you imagine?
It might not feel like real hair, but it will look beautiful and it will hold. Some people are lucky enough to have hair that does what you want it to with little effort and little to no product. The rest of us have to work a little harder!
Here are my favorite mousses.

Redken Full Frame

Sachajuan

Garnier
Go on now. Go get some curly hair!