If you`re reading this lines, this means that you already unzipped the downloaded package from Themeforest. Now, in the extracted package you will find a folder named “ARIVA Theme” and 2 archives: “ariva.zip” file and “ariva-child.zip” which are the archives for the wordpress theme and child theme of ARIVA.
If you plan to edit the theme files, like style.css, I`m suggesting you to install both ARIVA and ARIVA Child theme, then activate the child theme. Child themes are great because allow you to edit the theme and also lets you update the original theme in the future, without loosing your edited files work.
Read more about child themes and how to use them here: http://codex.wordpress.org/Child_Themes
You can install the themes in two ways:
FTP: Extract “ariva.zip” file and upload the “ariva” folder to /wp-content/themes/ folder on your server. If you want to use the child theme for future theme editing, extract “ariva-child.zip” file and upload “ariva-child” folder to /wp-content/themes/ folder on your server too.
WordPress: Navigate to Appearance -> Themes -> Add New -> Upload page. Select ariva.zip file. Press the Install Now button to upload and install the theme. Then do the same for the child theme as well, if you plan to use the child theme.
After uploading the theme, you have to activate it. Navigate to Appearence -> Themes page to activate the theme