Well first off, almost anyone can be an artist! Did you know the right side of the brain vs the left side of the brain thing is disproved? People can be both really good at numbers and be an artist. I mean I'm not a mathematician, but I've managed to stay in honors math while balancing my art skills and I've seen a lawyer that was really good at drawing people using just a pen.
I agree with MrRazot, you should start by doing cartoons. But the training method to get better is to copy what you see (Which is how I got better). So copy cartoons or anime, and do it often. In this way you get an idea of how to draw things with some influence from other artists, and with time you gain your own style. Make sure to be critical, but not so critical that it makes you feel bad and want to quit. You can use any trick you know to copy the art, {Like breaking down what you see into basic shapes} you just can't trace it because you don't really gain much from it unless you're studying how the person did something. (I suggest not going crazy and trying to copy colors and shadows. Which I was forced into doing. It's a bit too much for someone learning to do, instead try copying the form and using a black and white image. {Color can sometimes make things look different than it seems.})
Also, understand what you're drawing, and understand definite rules about things. For an example, almost all guys have a shoulder width of about three heads. (Not in cartoons and anime, but in IRL.) Tricks like these come in handy. Something like this:
Understanding and studying this is key. These things are called proprotions so you should find a few if you search up proportions of the human body (Or specific parts of the body online.)
Once you get really good at copying cartoons or anime, try real life. Just keep taking it another step forward, making it more difficult each time, but do it at your pace.
As you get better at copying you should find that drawing from your head is easier.
My friend actually was like you, he drew often cartoony stuff, but he was annoyed that he couldn't do realistic stuff. He asked our art teacher how to get better, and her response was to look at a real object or person and draw it/him/her.
In art, follow this quote: 'Do what you fear, and fear dissapears'. You're scared of backgrounds? Then draw backgrounds. Scared of portraits? Draw portraits.
As for watercolor, can't help you there. I cheat by using watercolor color pencils.
However if you really want to draw something from your head at the moment, here's what I suggest. Write down as much as you about your character as you can. In this way, you can focus on drawing the character and not so much on both drawing the character while trying to remember what you're drawing. (Which is what I often do for requests.) Use a multitude of references.