First, is what they have left you a 40 (or so) port router or a server? If they have left you a router which is wired to every lan wall socket, you're in business!
You will have to connect your server (I presume you already have one) to the router and then all the pcs. Provided an adsl internet connection has sufficient bandwidth for you all, simply connect the modem router to the 40 port router. If you want wireless, and you don't have an existing adsl wireless modem router, then buy a wireless access point and plug it into any of the lan ports.
If you don't currently have a server, think hard about your data back-up policy. What would happen to your business if any of the pcs failed? If you don't want or need a server, then consider using a pc (it doesn't have to be powerful) to store all your data rather than storing it on the hard disks of the individual pcs and institute a rigorous backup policy from that storage pc.