Start here...
http://en.wikipedia.org/wiki/DD-WRT
Read this and see if you're supported. Having the right model number may not be enough as each router has been revised over time/Try to find what version of DD-WRT you'll never and the flashing method.
http://www.dd-wrt.com/wiki/index.php/Supported_Devices
Then move on to their wiki for tweaks and tips on topics ranging for VPN to VLANing.
http://en.wikipedia.org/wiki/DD-WRT