Can i use Airport to let my PC lap top connect to internet through my Imac

If you haven't already purchased the AirPort Express Base Station (AX), I would recommend that you get a USB wireless adapter for the PC laptop instead. She can then connect the laptop wirelessly to the wireless network created by your iMac. Later, when you decide to get a dedicated wireless router, you could connect both computers to it.

In this scenario, you will want to take advantage of OS X's Internet Sharing feature. In this particular case, you will be sharing your iMac's Ethernet connection to the Internet modem out of the iMac built-in wireless (AirPort Card).

The following would be the basic steps to configure your iMac for Internet Sharing:

To setup for Internet Sharing (Wired to Wireless):
Setup the Network
Cable Modem > (Ethernet cable) > \[Ethernet port] Mac_#1 > (wireless) > PC Laptop

Enable Software Firewall - iMac
System Preferences > Security > Firewall
o Block all incoming connections (enabled)

Enable Internet Sharing - iMac
System Preferences > Sharing
o Select "Internet Sharing" from the options in the left column
o Share your connection from: Built-in Ethernet
o To computers using: AirPort (checked) (Note: Uncheck all other entries in the list.)

AirPort Options...
o Network Name: <anything you want>
o Channel: Automatic
o Enable encryption (using WEP) (optional)
o Password: (optional)
o Confirm Password: (optional)
o WEP Key Length: (optional)
o Click OK

o Select "Internet Sharing" again
o Click "Start" to start Internet Sharing
o Click Start