How do I know I have a wifi in my computer?

If you have WIIF (and correct me if I am wrong this looks like a desktop PC) you should have an aerial coming out the back of the machine.

You can double check by doing the following:
Open Network Connections by clicking the Start button , clicking Control Panel, clicking Network and Internet, clicking Network and Sharing Center, and then clicking Manage network connections. One will be called 'Local Area Connection' the other IF you have Wifi will be called 'Wireless Network Connection'

If you followed my steps then its an extra you don't have on your machine though you can easily purchase and install this.

Bring up a run command (Windows key + R)
type devmgmt.msc to load the device manager
look for Network Adapters, expand it (little triangle or plus at the left of the N)

Do you see a WiFi adapter? If you do, the computer is capable of wifi with it's existing hardware.

(*note it's possible, but unlikely you have a wifi adapter installed, but no driver. If this is the case, you will not see wifi under network adapters, just to be double sure, make sure network adapters is highlighted, and from the file menu, scan for hardware changes. A wifi adapter driver may now install).