If you are new to WordPress then you need to know how to install and use a new theme. Its very easy task and it can be done in many ways. However i will show you the most easy way to install a theme.
So, lets do it.
Login to your WordPress admin panel and navigate to Appearance > Themes . You will see there default themes if it is a fresh WordPress installation.
Now click on Add New at top and it will take you to the search page. You can search free themes here from right search box if you have no theme and if you have theme you can click on “Upload theme” and then it will allow you to select a zip file. Then Select your theme and it will be installed if uploaded successfully.
Then in the Appearance > themes page you will see your uploaded theme. Now hover your mouse over the theme you will get button to activate it. Click on “Activate“. and the theme will be activated.
and you are done! Let me know if you have any problem installing a WordPress theme.