Explanation
1. Click on "Appearance" and then click on "Customize" at your current template by default ("Twenty Fourteen").
2. Here you see the "Save" Button.
3. Edit the "Site Title" and "Tagline" for example ("Blog123", "Mynewblog").
4. Here you could change the header color and the background color.
5. If you want you could add a background image.
6. Select a static start page or your latest posts.
Now I show you who you could find a professional WordPress template, buy this and how to install it.
Sequence of events
Visit Themeforest
And follow the image introductions.
An explanation is on the end of the image.