Let's dumb this down a bit.
How's this: If you have a common Bestbuy bought type of router (ie: Linksys, D-Link, Netgear etc.. ) You will need to open a broswer and type the ip address of the router. This is most likely the way you're going to get in there to configure it. If you don't know the ip address of your router, it's likely the gateway address of any computer on your network.
To find the gateway address, Click Start -> Run type CMD and click OK. Then in the black Dos box, type ipconfig /all and hit enter. The gateway address will show up.
By default, a Linksys router's IP address right out of the box is 192.168.1.1 So in this case I'd open Internet Explorer and type:
http://192.168.1.1 and hit enter. If it's a Linksys router, the default username is left blank while the password is admin. Unless this is changed this is how it comes out of the box. Once you login to a Linksys router, the settings for changing to a static IP address are on the main page under the WAN section.
Each of these vendors of routers has instructions for this on their respective homepages. Or you can call support and they will walk you through this.
Note: If the password has been changed, no one knows what it is and the guy who set it has left the country, is locked up, shot dead or otherwise unavailable to give it to you, you can always reset the router back to factory defaults. But note (again) that you'll have to set it up again if you do this. It's not that hard but you'll need to find out things like the IP Address range on your network, or if there are any programs or access from the internet into your business and resetup those respectively.