Don't know where to start? Take art for your GCSE's/Highers (if you're in the uk). Grab some paper and a pencil, go nuts. Sketch something, sketch it again but in a different way/style to make it relate to a product. Make the alphabet out of every day items, food (alphabeti spagetti is cheating!) use dried leaves to make spray paint stencils for a fictional eco-company ad campaign.
The world is your creative lobster, you just need to think about what you want to achieve and then grind until you can execute it the same or as close to the view in your minds-eye
Personally I wouldn't even worry about making money right now. There is a lot to learn and when you start taking on clients you're going to be made aware of even more skills you didn't realise you needed. It's not the kind of thing you can attempt quickly and do well at. This is a career and will require some serious investment of your time.
I agree that you would be best off taking some kind of art course. You might not think it's relevant to logo design, for example, but it will help you to hone your creative abilities, which is definitely something required for logo design. From there you can look to take a graphic design course at college and then on to uni, if you so chose. However, you should be aware that you won't learn everything you need through traditional education. The best thing you can do is get into the habit of reading articles on graphic design which are plentiful and freely available online.
Another thing to consider is whether you want to become an art worker or a creative designer. Do you want to be the guy who takes orders from a non-designer and are paid for your abilities to use tools like Photoshop - or do you want to be the one in control of the entire project, building a brand from scratch and making all your own decisions. Also think about and research the differences between employment and freelancing. If you value creative freedom, freelancing might be a better fit for you but it's a much riskier path to take.
Squiddy has hit the nail on the head.
The only traditional education I had was school level art and a panchant for doodling vile cartoons of teachers untimely deaths in other lessons. I then hit the internet for tutorials, copied great designs to reverse engineer the thought process and ability. A big help to me is beta version kindles, or as we used to call them - Books, lots of books. If you learn enough, in time, you will earn more than enough
Beta version of kindles, I like that.
I like to think about projects in the following manner. Become successful at what you do, prioritise the service, process and final product and after that, the money will come to you by the boat load, as long as you don't screw something up!
Learn from tutorials . But don't ignore that "creativity" is the key to that all and let Photoshop do the rest! Best way to understand Graphic design get guidelines and tutorials from Youtube.com or other useful website's that have specially guide for graphics design...