If your computer is not having a builtin wireless LAN then, you might have a Wireless LAN USB adapter to get WiFi connections available. But if your adapter is not working because it does not have an installed driver, then, you first need to download the driver to make it work.
For this tutorial, I will install the driver for 802.11n wlan adapter. It has the hardware IDs of USB\VID_2001&PID_3308&REV_0200. The computer that I am using is Dell with OS of Windows 7 (64-bit).
3. Now, go to Start, right-click Computer and press Properties.
4. Click Device Manager.
5. Inside Device Manager, find 802.11n WLAN Adapter with exclamation point. Right-click on it and press Update Driver Software.
6. Then press Browse my computer for driver software. Locate and install driver software manually.
7. Just click the Browse button and find the file that you downloaded earlier. Select the driver that matches your current operating system and then click Next.
8. Wait for the installation to finish, and done!
9. To check whether the driver was successfully installed, view your Device Manager again. If 802.11n WLAN adapter with the exclamation is not there, then, your driver was installed and ready to be used. You can also check the Network Adapters, and you will see there the D-Link DWA-121 Wireless N USB Adapter.