AFAIK you can only put a single tag in the actual tag field, but they're arbitrary. You can use a tag which doesn't exist yet and the category will be created.
You can also put additional tags in your post body itself using hashtags, that's what I do. Just don't go totally overboard on it. Some people will put like, 50 random tags in their post, hoping to score lots of upvotes, and they just end up pissing people off instead. Make sure your tagging is relevant.
Edit: Also, I should point out for the record I'm hardly a Steemit whale or anything. I've only had one post which I would consider properly successful, but I've been here for about a week now, and I am seeing patterns in what works and what doesn't, even if I'm not very good at making use of that knowledge myself :)
OK, thats useful thanks, ill try something new and unique and see how that goes.
Its just funny how i first tested everything out and uploaded what i thought would be crap and i get a good payout.