Go to https://www.facebook.com/. This will open your Facebook News Feed if you're already logged in. If you aren't already logged into Facebook, enter your email address (or phone number) and password in the top-right side of the page, then click Log In. Scroll down until you see a list of people with birthdays today on the right side of the page, then find a person to whom you want to wish a happy birthday. Click the name of a person whose birthday is today. A pop-up window will appear. If your friend's birthday is today but it isn't showing up here: click the search bar at the top of the screen, enter your friend's name, click their profile image, and click the text box near the top of their page. Click the "Write on [Name]'s Timeline..." text box in the pop-up window, then type in the message that you want to post on their page. It's a blue button in the bottom-right corner of the pop-up window. Doing so will post your birthday message to the friend's wall.
Open Facebook. Review today's birthdays. Select a friend. Type in a birthday message. Click Post.