A theme is basically the icing on your cake, you can beautify it and make it look just how you want it to.
Click on Appearance, then Themes. You will see your list of themes here. There is the default theme which comes with WordPress. There are many free themes to choose from. When you see the theme you like you click on it and preview and Install.
When you go back to the theme menu click Activate and that becomes the theme on the website.
Click customize and play around with the menu to choose your options e.g footer and logos and save. Reload the website and see the changes. If you want to go back to the original theme just go back and activate it.
themeforest.net is a great resource for buying themes if you are not happy with the free ones. If you would like to ask questions about a theme you can always contact the developer for help with certain customisation issues