Hi
Follow the instructions here:
If you don't have access to computer with iTunes:
- See whether you can borrow one from a friend.
- Visit an Apple Retail Store: Find a Store
- Or visit an Apple Authorised Service Provider: Apple Authorized Service Providers
If you have not yet been able to use the iPhone, keep in mind that it may be protected by Activation Lock. If so, the previous owner will need to remove the lock before you can set up the iPhone as your own: