Here, are some steps explain you how to get know who is using your wifi or using wifi by stealing your password
click here and Install Who Is On My WiFi (also known as WIOMW).
After opening this application, follow some tutorial steps given, which can initiate first network scan.
Now you can config port range that able to scan, but if you are starter, don’t need to mess with other settings.
Now you can see a list of IP addresses appear in the app window after scan. Each one can be identified by MAC address and local IP (one your router/switch/modem ).
Now get ready to identifying the devices on your network.If there are many connections , it might be easy to turn on Wi-Fi one device at once so you can label it.Label just click on your Description box on the appropriate line, type your name and then click Save in the topof the right-hand corner. Know where to find local IP / MAC address, you can use these as a references for the labeling process. For example>> tapping the currently connected Wi-Fi network on Android will display local IP.
STEP 4 (Opt):
Now if you found that someone is indeed who leeching your Wi-Fi, you can change your wireless password/wifi password. On your personal wireless router, just log in using your credentials . When you using a moderm/router combo from your ISP, the process may become more involved. Search to make and model of modem to find the default login . If you are not able to log in, contact to your ISP provider and reset your password.