Why reinvent the wheel ?
Buy a wireless router designed to work with an external modem -- you'll need to specify that because most sold these days (for DSL customers anyway) are integrated modem/router/wireless units*.
In fact, why not get one of those -- setting up the connection to the ISP should be straightforward (note down the existing modem settings and duplicate them in the WAN section of the router).
*so they tend to be cheap -- in fact there are thousands thrown away every year as users switch suppliers and get a new wireless router free.