I have a siemens 4200 adsl modem i had a n...

Hello Mate,
the siemens speedstream 4200 is a DSL modem/Router which doesn't have wireless connection. So even tough if you have a computer equiped with a wireless adapter, you will not be able to connect wirelessly because the siemens 4200 doesn't act as wireless access point.
You may need next time to check first if the modem/router to buy support wireless (wifi g or n) and make sure that it's not only a modem, it should be also a router.
i attached here some references for you from the top brands:
1. Linksys DSL modem/Router with wireless N: http://www.linksysbycisco.com/MEA/en/products/WAG120N
2. Linksys DSL modem/Router with Wireless G: http://www.linksysbycisco.com/ANZ/en/products/WAG54G
You can find other brands as Dlink, Netgear ...
Best Regards,