Installing a theme is one of the basic setup in starting a Blog. A theme which suite your blog topic and other benefits. You should avoid random theme for any usual site because most of them are risky for your blog and are premium themes which are stolen form different sites this which is often a demo of the Original theme.
Installing WordPress Theme
First of all go to Appearence >> Theme and click on Add New
Second click on Upload Theme Or you can choose from the 1000 of theme its free.
Third Choose the theme you downloaded, Remember it is in zip format.
After uploading now finally activate it.