I did very badly in college, and majored in American History. About three years ago, I quit my job and learned how to code. I was focused on machine learning and data analysis. I faced the same problem you did -- no way to prove I had any skills.
The way to stand out to employers in your field is to do things that are publicly visible. It's hard to judge someone's talent from a resume, but it's easy from a github repo.
Some ideas for you:
* Contribute to open source projects in the space you're interested in.
* Start a blog. You'd be surprised by how much sharing some of your own experience can help others, and it can raise your profile a lot.
* Lots of companies offer bug bounties -- try to find some vulnerabilities and collect some. Blog about your experiences. I don't know how hard this is, though.
* Make cool projects and share them on github. Blog about them. Post them on HN.
* Go to meetups in your area on topics that interest you. Meet people. Make cool projects with them.
* Go to hackathons if you can. They're a great way to meet people and find interesting projects.
* See if you can take on a small part-time role helping out at a startup in your field.
As for what's in demand, take your pick. I've talked to companies having trouble hiring web developers, mobile developers, and data scientists. Any of them is a good choice (again, don't know anything about IT security demand).