Hi and Welcome! A hub is an article in it's own right, though you may decide you want to write several on one theme and group them together (using the group function in "my account").
A new hub can be whatever you want it to be depending on what you are writing about, but in general each hub is topic dependent. So if you are writing about a personal experience say on pregnancy, that would go in the pregnancy category. If you write another hub on your experience of holidays in the maldives that would go in the travel category. You'll find categories for every topic you want to write on.
I would suggest that you make sure your hubs do not have days/dates as the titles, unless you are writing purely for personal satisfaction, because no one will find them.
Have a look at some other hubbers hubs and I'm sure you will get the idea in no time! :)