There are tons of tutorials on how to add an announcement banner using PowerShell or custom web parts, but why go through all that when you can use plain-old CSS to get the job done via the UI?

If you browser to the Master Page section within the Site Settings area, there is an option titled “Alternate CSS URL”. Within the file used here, you can easily add the following CSS to inject a quick announcement to all of your SharePoint pages:

#suiteBarDelta:before {
    content: "[Enter desired text here]";
    display: block;
    padding: 10px;
    color: #000000;
    background-color: #ff8383;
}

Apply your changes and refresh the page to see your announcement on display.