There is no need to sub first. I would be careful of anyone who says to you that you cannot be a Domme until you have subbed first, especially not if they suggest you be their sub. This is a classic scam for getting a 'free sub' often used by wannabe Doms and you won't necessarily learn anything abot being a Domme.

Bottoming for a while *may* help you get an idea of what sort of things you would like to do to your own subs when you get them - give you an empathy for the sub life - but is not essential. Instead, I would suggest that, after you have done all the reading and stuff mentioned above, you see if anyone is willing to help you out by letting you watch them practise and maybe help out. Best way to learn is hands on experience with supervision to make sure you don't do anything wrong. Your local munch may be the best place to find a Domme to work with on this.

You don't say if you want to be a lifestyle Domme or a Pro Domme... I think all the above assumes a lifestyle Domme.