First of all make sure that you've specified key&secret for your new site, you can generate it on una.io site in Account > Keys & Secrets, then you can insert it on new your site in Account > Studio > Setting > GENERAL.

After above is done you can install new modules from Account > Studio > Apps Market > Storefront and configure your site.

Here are some of docs which will help you to get started:

https://github.com/unaio/una/wiki/Add-Apps

https://github.com/unaio/una/wiki/System-Apps

https://github.com/unaio/una/wiki/Studio

And other docs in Configurations section