You write a lot and constantly. An idea pops into your head, stop what you're doing and write.
Also share your writing with people, use apps or websites like wattpad in which you can publish your work and have people read it and tell you what they think. Take their criticism and use it to better your work.
Go out of your comfort zone and write other genres, you write horror, rewrite it as romance, a murder mystery or a drama. Go on tumblr or reddit and find writing prompts that interest you and create something out of them.
So if you want to be a better writer, experiment, don't stop writing and let others review your work and use their feedback.