Both PC's\laptops need to be in the same workgroup.
Right click My Computer, choose properties and then click on the computer name tab and change as required. Secondly to access another PC on the network you will need to share a folder or printer for example.
If you share the printer, right click on the printer on the PC that it is intsalled on and choose sharing. On the PC where you want to access the printer over the network, choose to add a priner, choose network printer, next and choose the second option. For example; \\laptop\epson this would indicate that laptop's name is laptop and that the printer is shared as epson...
Don't worry about IP addresses as your router will take care of this. Secondly do you have any firewalls installed on any of the PC's? If so they will probably cause a problem an you will need to configure them for local network access.