As this is a wireless router, one of the first things you should do is ensure you have WPA / PSK encryption enabled, usually called PSK-Personal at least on Linksys Wi-Fi equipment. If you have encryption disabled or you use WEP (easily broken into with tools widely available), there is a good chance that you also have neighbours illegally using your connection for downloading.
Go into the router's web interface, into the 'Administration' tab and into 'Management'. Set 'UPnP' to disabled. This will prevent P2P software (BitTorrent, Limewire, etc.) from automatically configuring incoming ports. While this will not stop Torrent traffic, it will certainly cripple it and in many cases make their downloads painfully slow. Save the changes.
Next, go into the 'Applications & Gaming' tab. Go into the 'Single Port Forwarding' section and if you see any ports set up there that you know you have not set up, remove the 'Enabled' tick and save the changes. By default, no ports are forwarded, so if you have not set up any ports to forward before and see values set up in here, then these were set up by someone else. Repeat for the 'Port Range Forwarding' and 'Port Range Triggering' sections.
Go into the 'QoS' section within the 'Applications & Gaming' tab. If you have a wired connection to your router, follow the first step, otherwise the second: This will give traffic priority to your PC.
Wired: Tick 'Enabled' for 'Internet Access Priority', select 'Ethernet Port' for 'Category', the port your PC is connected to for 'Ethernet', set 'High' for 'Priority' and click 'Add'. Then save the settings.
Wireless: Tick 'Enabled' for 'Internet Access Priority', select 'MAC Address' for 'Category', enter "My PC" for "Enter a Name", enter the MAC address shown for the "My Current PC's MAC Address:" info shown on the screen into "MAC Address", set 'High' for 'Priority' and click 'Add'. Then save the settings.
Finally, go into the 'Administration' tab, into the 'Management' section and type in a new password for your router and save the settings. This will prevent your flat mates from making their own changes.
If your flat mates complain about your ISP's performance or port forwarding issues, tell them if they want better access to get their own Internet connection.