WordPress posts can be assigned to categories and tags. These are taxonomies that help users filter your posts, and they're worth thinking about before you create too many posts. Having said that, they're not compulsory, although most blogs end up using one or the other, or both.
Categories vs Tags
When all said and done, there isn't a great deal of difference between categories and tags from a user perspective. If someone who's particularly interested in fish visits your site, it doesn't matter much to them whether they use a category or tag to filter your posts for fish-related information.
As far as WordPress is concerned there is a difference, though. Categories can be hierarchical, whereas tags cannot. Categoroes can have subcategories but tags cannot have sub-tags.
How you organise these things depends on the sort of site you have and your aims for it in the longer term, but it's worth thinking about this up-front. Recategorising or retagging a site somewhere down the line is tedious and could be detrimental to your SEO (Search Engine Optimisation).
My advice is this: if you're just going to use one, use tags. They're simpler and more universally recognised. If you're going to use both, make sure they don't do the same thing. It would be pointless, for example, to have a category for fish and a tag for fish. There needs to be some separation.
One way you might do it is to use categories as higher level divisions, and tags as lower level divisions. You might, for example, have a category for Fish, a subcategory for River Fish and then a tag for Trout.
Another way to do it is to use categories and tags for different types of information. Maybe categories would be things like News or Opinion, describing the type of post, and maybe tags would be things like Fish or Goats describing what the post is about.
You can manage categories from the Posts menu item in WordPress admin. Select the Categories submenu item. You'll then see a screen like this:
You can add new categories by filling out the fields on the left (1) and pressing the Add Category button (2).
You can thereafter manage your categories by selecting them from the list on the right (3).
You can manage tags from the Posts menu item in WordPress admin. Select the Tags submenu item.
The process is exactly the same as for managing categories, above, except that tags have no option to enter a 'Parent' (because tags, unlike categories, are not hierarchical).