Hopefully your new DSL modem can connect by ethernet because if it can only connect by USB you are going to have difficulties.  Assuming that the new DSL modem does or can connect to your desktop computer by etherent, you will need to purchase a wireless cable/dsl router.  Such routers will connect to the dsl modem through an ethernet port.  Up to 4 computers can connect to the router through a wired ethernet port (most routers have 4 ethernet ports or LAN ports), and many more computers can connect by wireless.  Although you could connect your current desktop by wireless, there is really no need to do this because it is probably located close to the dsl modem and would still be located near the router after installation.  Furthermore, it is always a good idea to connect at least one computer by a wired connection so that you can configure and troubleshoot the wireless connection when it doesn't work. 

You have a wide choice of routers and most are relatively inexpensive.  You should stay away from those that offer speed boost technology because that would require that the router and all wireless network cards be from the same manufacturer in order for them to use the high speed.  Note that even the slowest routers (802.11b) are still much faster than your dsl connection, so that unless you plan to transfer many large files between the computers, the 802.11b or faster 802.11g routers should be just fine.

Steve