If you're not sure whether to make a page or a post, here's a quick comparison:
Pages are pretty much what they sound like: basic, standalone web pages. If you're in doubt, it might make sense to default to a page for your content. Another way to think about it is: if you plan to use WordPress like a regular website, mostly you should be making pages.
Posts are like blog posts -- since WordPress started out as a blogging platform, posts are one of the main elements of a WordPress site. If you want to publish new content periodically and have the newest stuff show up first on your website, consider making posts.
Posts also have categories, which allows you to sort them by type for users (or lets your site's users search for more like whatever they're reading). You can even create menu links to post categories, which can really come in handy. Ultimately, if you think you're going to use WordPress like a blog, mostly you should be making pages.
To make a new WordPress page: