The most common reason that the Site Title doesn't show is that the theme you have selected requires the Title at the top be a Logo Image. Some themes display the text and others require a logo.
Many themes do not show the tagline, but we can often unhide this with some CSS magic - just let us know and we'll be happy to help!
If your theme requires a logo, then you can either select a different theme or use one of these services to create an image logo.
(free online image editor) http://apps.pixlr.com/editor/
Many of the following are free to design your logo, then you pay for the final product...
You can even try a service like www.fiverr.com
NOTE: We do not recommend a full-width header for any theme. This is a very outdated design and most newer themes do not support it. If you have an image that you would like to feature, newer themes will often have "widgetized" home pages, where you can feature content.