Step 1: Log into your router
To find out what devices are currently soaking up that sweet, sweet speedy signal, you'll need to log into your router. Type the device's gateway IP address into your browser. The two most popular router brands, Cisco and Linksys, share a default address of 192.168.1.1 -- otherwise, you should be able to find yours on this page.
You'll be prompted to enter the username and password, and unless you switched things up when you first installed the router, your username is most likely still the default "admin" and your password is either "password," the router's serial number (which you'll find on the physical router itself), or blank.