How to make your own offensive system?

And also how to draw up better plays? I wanna start coaching but Im only good at teaching and draw up plays a bit.

You can make your own offensive system by choosing a formation and rules for your offense. That's a motion offense and what makes it "yours"... it's based on the rules you choose. You can read more about how to do that here:
http://www.breakthroughbasketball.com/pr/motionoffense.html

For "set plays", that takes experience to learn what types of action works and doesn't work. With that said, almost all set plays are based on fundamental concepts like.... spacing, ball reversals, mis-direction, screens, cuts, etc. You can read more about set plays here:
http://www.breakthroughbasketball.com/articles/16-set-play-tips.html

And more about offense here:
http://www.breakthroughbasketball.com/Offense/offensiveTheory.html