Google analytics is a free and useful tool for getting information about your site visitors, and there is little reason not to set it up on your site before launch.
You will need a GA account, and know it's account number. Install the GA module linked, and enter the account number into /admin/settings/googleanalytics and customize if desired.
When using GA in Germany you have to publish "datenschutzrechtliche Hinweise" on your site to inform users, that some of their data - e.g. IP-adress - will be tracked. Normally you set up a text block in your imprint. Using GA is maybe not conform with contemporary requirements of protection of private data.
