The 2 main mistakes i see players make in dominion are having no idea where to go and when. And the other is how to gank effectively.
Please sticky this well made guide:
https://www.youtube.com/watch?v=-UoNJk3aOng
Id like to add that, if you have no idea what to do, the safest option is just to feint a heavy and see how the other player reacts.
Feel free to edit this post to your liking before you sticky it.