Get ready, this is a long comment. I got sort of carried away, but I hope some of my rambling helps you. Please excuse any typos/grammar errors—this is just far too long to proofread ;)

First of all, don't tell people you're 18. Since this is a pretty good community and I want to help you out, I'll go ahead and be a tiny bit hypocritical here and tell you that I'm 18 as well. When you're doing freelance work, if a potential client sees that you are 18 years old, it can quite feasibly end up being a turnoff for that potential client—regardless of how nice your work is. The excuse many people give for giving their age is that it makes them feel like a young-gun, an up-and-comer, an impressive person (now please don't think I'm saying this is you—it likely isn't—but a lot of young designers make this mistake). That doesn't work. Just let your work speak for itself, do your best, try to be nice to people (even when they are complete assholes to you), and hustle like hell. You'll get far in your career that way.

Ok, on to the next thing. Branding isn't just a visual identity, or your portfolio, or even just what people think of you. It's a combination of all those things, as well as every interaction you have online and in person, and how you treat people and respond to problems between you and your customers. A large portion of a personal brand is built upon your ethics, your personality, and the way you interact with the industry and your customers (in the corporate branding world, this is called corporate social responsibility—but in this case let's just call it personal social responsibility). Be friendly when you're writing copy in your portfolio—add some personality and don't try to be overtly professional in the way you speak and present yourself. Professionalism will get you far in life, but personality and charisma can get you that client that makes your career. Treat clients the way you'd want to be treated. This seems obvious, but it's something many designers forget to put into practice.

Next I'd say do what Pasquale said (he's a cool guy so heed his advice—I've never met him or really interacted with him but he knows his stuff and he's respected in the industry) and find what you're good at, then make it a point to show people that thing. As a rule of thumb, the work you showcase is the work you'll get. If you show art in your portfolio, you'll get client who want art. If you show paper planes in your portfolio you may not get many clients, but the ones you do get will ask for paper planes. Show people what you're good at, even if you don't have a lot of that work. Don't just show work for the sake of having a larger, more expansive portfolio. But don't only show people what you're "good" at; show people what you love to do as well. If you are insanely passionate about paper planes, put some damn paper planes in your portfolio, or blog about paper planes, or just send paper planes to people you admire and see if they take notice of you. Passion is infectious, and it is very noticeable.

The last thing I'll leave you with is that you should make sure to put yourself out there. I forget where this quote is from, but a designer said in an interview or something a couple years back that you need to "show up every day". Keep showing up. Every single day you have to hustle like hell and make sure people know that you exist, that you're out there in the world. That you're doing what you love, and you want to help other people by doing whatever that is. Post some blog posts—they don't have to be long think-pieces or even related to design—just post some cool stuff that you like and share it with people who also like that stuff. Make sure you network with other young designers—they are going to be your peers for your entire career so it's important to start these relationships early on. Feel free to email me any time.

Basically, just put yourself out there in the world; show people what you're good at and what you love to do, that you're passionate; be responsible, ethical, and kind; and be charismatic, make sure people know that you're a person, not just another business.