First of all you need one of these click here
You will need to open your pc to fit the card.
If you do not feel comfortable opening your pc then get one of these click here
With one of these fitted yes you will be able to connect both computers to the internet at the same time. But you will have to read your router user manual on connecting your wireless card to the network.
As for connecting to the pc from the laptop you will need to set up a home network (please browse the forum on how to do this).
If the computer breaks down or is turned off and the file that you need is on there then you will be unable to access it. The only way round this is to get a very expensive server unit.
Hope this helps.