Ok, there's a couple of directions you could take:
- You could draw the real heads, that will get you the best overall result in every art style, but it will be a long journey and you will pick up things that will diminish your skill at anime drawing - for example, in the anime head the eye sits higher than in a normal head, the nose has a completely different shape, etc.
- You could just copy anime, and that will get you the quicker results. You will still need to do some real world studies, mainly realistic skeletal structure and "Anime" skeletal structure (basically, get how the skull works, then go see some anime illustrations and try to define the skull underneath them). With this method you will get shit done in no time if you put in the effort, but you'll be able to draw basically only anime
- A mix in any % of the 2 above