You will need to check that the computer has a wireless card. If you check the back of the computer it may have an aerial coming out of the back if not you will need to check the specifications of the computer or failing that you need to check the computer hardware.
You can do that by going to the start button then control panel then device manager then scroll down and look for network adaptors then click on that and look to see if there is one that says wireless adaptor.
If you have a wireless card you then follow the guide in this link
http://bt.custhelp.com/app/answers/detail/a_id/9692/~/how-to-set-up-a-wireless-connection
If you don't have a wireless card you would need to buy a wireless dongle and after plugging it in to the computer you would connect as above.
As regards the HP printer it should have come with instructions how to connect it to a router. You need to follow them. If you don't have the instructions you should go to the HP website and you will find them there.
