Take some life drawing classes. They may not seem to have a connection to what you would like to do but they would really help you to get to where you wanna go to. First of all it will help you draw more loosely. Then it will help you see how you can further your journey to making cartoons. All cartoonists do some sort of life drawings to enhance their drawing skills and not just doodle every chance you get. 😉
If you ask around or look around, you can probably find little to no cost life drawing classes for your needs. Also the schedules are usually pretty flexible too. Things you may need: Don't give up enthusiasm even if you feel not up to par, some conte sticks
, some black carbon sticks, various drawing pencils,a large clip pad and a news print pad. Other bonus things is up to you.
Also you may wanna check this guy out. http://gurneyjourney.blogspot.com/
He is very good at the example of real life drawings to imaginary worlds too.