Hi luifer,
First you have to ask to your Internet Service Provider about your modem, ask them If is router as well... or If is wireless.
If it isn't a router or wireless then you have to buy a wireless router.
In case that your modem is wireless or do the function of a router then you won't need a wireless router. Almost every modem has 2 ports in some cases 4 ports by default.
If you don't want to buy a wireless router then you can connect your laptop using cable UTP Cat 5 or 6, however you'll need to configure the drivers of your modem If you have the CD that cames with your Modem you'll be fine. [This is because one computer will be connected using an USB cable and the other a RJ45 cable (UTP) the cable that seems like a cable phone.
A wireless router is a device that you need in order to connect a computer(s) to a LAN and Internet.
A good option is to get it in BestBuy.com, Frys.com or tigerdirect.com.
A router is a device that extracts the destination of a packet it receives, selects the best path to that destination, and forwards data packets to the next device along this path. They connect networks together; a LAN to a WAN for example, to access the Internet.
Good luck!
Greetings,
Fernando R
Security Information Engineer
Vancouver, British Columbia, Canada