Kala is right on the money - practice. I used to doodle all the time when I was in school...I still have my notebooks, and the pages are half writing, and half drawings. Then what I started doing was looking at toy figures and drawing them. I highly suggest you grab a human anatomy book and draw the human body as much as you can. Draw eyes, hands, muscles, ears, mouths, feet, calves, and then start drawing them together as a body. Make sure to draw your outline first so that you see that the proportions are right.
Then if you want to draw Preds, look at artwork, sculpts, and everything you can get your eyes on, and start doodling. Draw until everything in your head is out on the paper...and then go back, get more ideas of stuff you think would look cool, and start over.
Its a long and slow process to get from drawing to being good at it. It took me years and years, and I'm kicking myself for not drawing for years in between. You have to have a passion for it and the drive for it. Just remember to practice as often as you can
