Go to Ross + Rachel
Go to Chandler and Monica Photo Album
Chandler and Monica has always been close(how do you think she found out about the third nipple?). Chandler has always been there for her (He peed on her to take out the wrath of the Jellyfish)and thats how they started. Also in TOW the Jellyfish, Chandler wanted to be Monica's boyfirend.
Monica and Chandler was at London for Ross's wedding with Emily. Monica was having a really bad time and Chandler felt sorry for her. The next morning, you see Chan and Mon together in bed talking about what they did. They both liked it so they kept doing it. But they never told anyone that they were doing it. A couple of weeks later, Joey found out, then Rachel, then Phoebe, and then Ross. That's when they actually became couples.
