You need a Shopify store. If you don't have a shopify store, you can go to Shopify and create your store. We also recommend that you should read the Shopify Manual.It is an extremely well written and thorough guide to understanding the Shopify platform, written by Shopify themselves. It will help with understanding many aspects of the customization of your theme as well as using Shopify in general.
Purchase and download the theme from themeforest.net
Once the theme has been downloaded to your computer, you are now ready to install it to your Shopify store