The more you sketch, practice and draw, the better you get. You need to observe how other artists achieve the look they use, what do they do, how do they handle the proportions, shading, etc. You learn by observation.
Most artists I know never took an art class outside of high school, they do it all on their own. Personally I found my 'style' changing 4 times in 10 years, so you do learn and grow as you go.
By learning basic human proportions, as a cartoonist, you can then exaggerate what you need and have it look pleasing. Learn the rules so you can break them artistically.
And get yourself a stack of scrap paper. Sketch!!!! Keep the best 10% each week and toss the rest. At the end of the year go through your saved sketches and you'll see how much you've progressed with your skill level.