Setting up a home wifi network

Hi, I decided that having signed up for adsl now would be a good time to set up a basic home wifi network, unfortunately I seem to have rapidly gotten over my head, any help would be much apreciated.

I have 512k capped adsl (with USB modem I got from the ISP) I have one desktop linked to this in my upstairs bedroom, I have one laptop I usually hook up to this also, finally I have a second desktop out in an office/shed in my back garden with a laser printer running off the foul dial-up.

What I want to (eventually ) accomplish is to able to access the broadband from both desktops and the laptop anywhere in the house. I'd also love to get file and print sharing going accross all the pc's wirelessly eg be able to pull a file off the upstairs PC on my laptop and print it out in the garden office and so on.

What i'm stuck on is this USB modem in part, i cant see how to integrate it into a wireless system, do i need a new modem? Also while wireless access points seem devoted to internet sharing, can I share files accross all my computers without trailing any wires accross the garden and whatnot?

I really would apreciate any help as i'm totally lost, cheers eveyone .