Once you've made a lot of crochet items from somebody else's pattern, you start to make changes to the pattern so that it becomes your version instead of the writer's. That's great!
You can begin to write down how you did that, to come up with your version of the pattern in print form. When you write crochet patterns long enough, you begin to get the feel of how its done. But to begin with, you will find yourself writing your patterns using the same words you found written by someone else. But you made changes to their pattern, so you didn't exactly commit a crime. But you still can't legally sell that pattern until you can write down how its done without copying someone else's work. You have to change a minimum of 40% in order for it to be anywhere near legal to do that and offer it up for sale. And it can't look like a knock off of the finished item, either. This is important!
When you can come up with your very own pattern based on the techniques you've learned crocheting, and put it down in your very own words, and you know it is yours and yours alone, then and only then can you put your pattern out there for the world to see.
Put your first patterns up on the internet for free in a public place, such as crochetme.com. You won't get any money for it, but you might get a lot of praise and that's what you want. People will tell you if you made a mistake. Go ahead and fix that mistake. Don't waste time getting your feeling hurt or worse--getting all mad about it.
When you are good enough at the free patterns that people start asking you if it's really your pattern....THAT'S when you are ready to start selling your patterns.