In order to sync Nokia Lumia with windows 7 on WiFi , just follow step by step procedure:

1. you will need to use your USB cable for connecting Phone with PC. Connect it.
2. Launch Zune on your system.
3. Go to Settings and click on Wireless Sync options.
4. Now, go through the instructions shown on screen to complete the settings and you are done.

Note : during the entire process your phone and system should be connected over same WiFi.