Hair that's been straightened for years needs a pretty radical cut at first, so that's the real first step. I'm not trying to scare you, but as long as you've got burnt and dried-out ends, your curls are never going to look good even if you ditch the blow dryer. I'd say find a hairdresser who has experience with natural curly hair (this matters, it's not the same as someone who just cuts straight hair) and tell them the whole story: how long you've been straightening it, how damaged it is. They'll probably recommend cutting off at least 5-10 centimeters, sometimes more. Yeah, it hurts to see it fall, but it's honestly the best thing you can do for your hair.
Once you get the cut, the transition is about products and patience. Forget regular shampoo - a curl-specific one makes a huge difference (they're usually more moisturizing). When you wash it, do it with your head upside down, apply generous conditioner and comb through with your fingers or a wide-tooth comb while it's wet. Some "leave-in" products are going to be a lifesaver because dry curls tend to frizz without something to seal them. And here's the hard part: you're going to have to let it air dry or use a diffuser on the blow dryer, no regular blow dryer or flat iron.
Mentally prepare yourself that the first few months are going to look weird, like a hybrid between curly and flattened hair while it recovers from all the accumulated damage. Some curls are going to appear or change shape as they grow out. But if you give it time and the right products, in about six months you're going to start seeing a real difference. And honestly, once you see your actual curly hair looking good, it's hard to go back.