It would be helpful if we knew what OS you are running.
If it's Windows 7, you should be able to do this from the Network and Sharing Center.
Click on "Set up a new connection or network" and then select "create a new ad hoc wireless network". This creates a new virtual network in the wireless networks list - make sure that you're connected to it.
Now you need to enable internet sharing on the adapter that you use to connect to the internet. In Network and Sharing Center, click on Change Adapter Settings. Right click on the internetted network card and select properties. Finally, select "Allow other network users to connect through this computer's internet connection."
You should be good to go at this point.