Minecraft wiki entry for Setting_up_a_server, associated text: Windows
Press ⊞ Win + R; this should be up to the "Run" dialog box. Type cmd and hit ↵ Enter. This should open a command window with a black background. From there, type ipconfig and press ↵ Enter. You should be given a list of text. Scroll up to "Wireless LAN" (if using wireless) or "Ethernet" (if using a wired connection), and look at "IPv4 address". To the right of this should be a string of numbers (of the form xxx.xxx.xxx.xxx). Copy this down by right-clicking the window and selecting "Mark", then highlight the area and hit Enter. Don't copy any parentheses or letters.