I assume that's a
Siemens SL2-141
There is a manual for that device here:
http://www.iad.cz/prez/ADSL_SL2-141_User_Manual.pdf
If the device is not connected to your computer, follow the directions in the manual. I have to start somewhere, so I'll assume that it is connected and working properly.
On a computer that is connected with an Ethernet cable to the router, open a browser (Internet Explorer) and enter the address, 192.168.1.1. A challenge box will appear requesting a username and password. The default username is "admin" and the password is also "admin". If anyone has changed the address, the username, or the password, you'll have to check with whomever did that.
Once you connect to the router, click on the "Wireless" menu entry.
On the Wireless screen, check the box labeled "Enable Wireless Network". Where it says "Wireless Network Name (SSID)" enter a name for your network. This name will be "broadcast" and anyone in the area with a computer and a wireless card can see it. Make it clear enough for you, but don't make it obvious who owns the network (gotta keep 'em guessing).
You'll probably want the "Transmission mode" set to "802.11g only" if the computers you wish to connect all are 802.11g compatible. Most recent cards are. Otherwise, you can also leave it at "Mixed Mode". That will allow 802.11g and 802.11b cards to connect.
Leave other settings in their default or auto states.
Under "wireless security", I suggest you set it to "WPA-PSK". PSK means "pre-shared key". Set the "Format" to "ASCII Characters" and enter a long string of characters into the "Pre-Shared Key" field. It should be something that you can remember (you can always come back and look at it if you forget), and preferably will have letters, numerals, and special characters. Something like "I always eat dinner at 8:00 in the evening when I'm sick" might be adequate.
Once you've "applied" those settings, you can get onto the computer you're trying to connect to the network.
On that computer, I'm assuming that software came with the wireless card. In any case, you have to set up wireless on that computer. How you do that depends on the card in the computer (or is wireless built-in) and the software that comes with the card. Also, the version of Windows is important. Are you using Windows XP, Windows Vista, or something else?
In Windows XP, if you are using only Windows built-in software, usually the card will be recognized when you install it. If a "Wireless Network Connection" appears under "Network Connections" in Control Panel, double-click on its icon and then click on "Network Connections'. You should see an entry with the SSID that you entered into the router. Click on that entry and click "Connect". If everything is correct, you will be prompted for the Pre-Shared Key that you entered into the router. You will then be connected to the network.
You should be able to surf the 'net. If you want to connect to other computers, that's another story.