Wordpress has a very helpful page called “Get Published” that walks through some of the basics of writing posts and pages.
You can add a new post or page either with the toolbar, by clicking on the + sign and then choosing a post or page …
… or with the Posts > Add New and Pages > Add New buttons in the sidebar.
The visual editor works much like any other text editor you might have used before. You enter a title for the page or post in the title box. You have a row or two of formatting buttons along the top of your text editor window (or more, if you've installed plugins like WP Edit to expand your choices of formatting buttons).
To insert a link into your post or page, just highlight the text that you want to be the anchor for the link and then click on the button that looks like links in a chain.
An insert/edit link dialog box will open up. In the URL box, enter the address that you want to link to and then click on the Add Link button. You can choose whether the link will open in the current window or in another tab by checking the box or not.