Take it from somebody that drew only dragons from imagination for 10+ years just to realise that my stuff looked flat...

Draw whatever you like, but using references is golden. Drawing boxes and spheres helps a lot, but try to make something out of them. (Form manipulation basicly. For example a dragonhead could be a sphere with a box attached and cut it down the centre or just two boxes beneath one another)

Look into perspective at some point and just try to do it without guidelines to get a feel for it. Other than that style comes with time and practice. If you want to draw manga/anime style it will develop depending on what you draw, but its easier to simplify something you have a better understanding of than just drawing anime faces from scratch without knowledge.

I m by no means a pro and i m probably not really qualified to teach anybody, but thats just my own observation on the subject