Surely you have already installed WordPress on your server. If you are new to WordPress and have problems installing it, you might want to check out this link:
Unzip the file you download after the purchase. The following files and folders will be in the unzipped file:
1. Via FTP client
“Emerald”, unzip it and upload the folder to the main WordPress installation folder » wp-content » themes. Please note that if you prefer this way of installation, you have to make sure that the main files of the theme are contained just within one folder of the themes folder. The right way to locate your root theme files is for example themes/Emerald but not themes/Emerald/Emerald.
2. Via the built in WordPress theme uploader (recommended)
“Emerald”. After this you have to go in the admin panel » Appearance » Themes, click on the Install Themes tab and in this section in the top there is a link “Upload”. After you click the “Upload” link, a new page will be opened where you will be asked to find the Emerald.zip file on your local file system.
Dashboard»Appearance » Themes. In this section you should see the theme with a title Emerald. Just click on Activate button and your theme should get activated.