DDoS attacks are very easy to setup. I could show you in 5 minutes how to do it. It takes a few days/weeks for you to infect enough computers to act as pawns in your DoS attack depending on the size of your attack.
The basic premise is this. Scan the internet for vulnerabilities. Infect vulnerable computers with a trojan that scans the internet. Repeat. Exponential growth. These infected computers are known as "bots".
The trojan is one that is able to receive instructions (a variety of ways depending on how you set it up, such as getting info from an anonymous IRC channel) and execute it.
When you have enough bots at your disposal, go ahead and execute any command you want. You can target the web server itself, the router, the upstream ISP, etc.