I'm new at Magento and installed magento 2. Now I want to add themes through extensions. Please tell me how can I install theme? When I got to magento market then i get extension key. What I do of Extension Key?
The Marketplace (Component Manager) for Magento 2 is under development yet: https://mage2.pro/t/985
For now you should install the themes and extensions manually, following an instruction provided by the developer.