If you haven't taken any of the classes yet, Jake Parkers "How to Draw Everything" is a great place to start and there are some really good assignments to download and practice. Especially if you're struggling to get started. Then move on to the other foundational classes.
As for practicing on your own, one suggestion is start compiling images from artists that you like and would like to emulate. Pinterest is absolutely great for this. Trace their work, try to recreate it and elements of it in your own drawings. Draw things your'e interested in. Make a list of things your would like to draw well {example: animals, cars, robots, monsters etc.) and then get to it.
Another suggestion might be , Find every tutorial you can and practice the techniques until you have them down really well. Again Pinterest is a great resource for this as is Youtube. But the best one is of course SVSlearn.
Everything will be frustrating and look bad at first, but don't get down on yourself. Every great artist started there too.
Hope that helps