You first start with gathering specifications as to what features you should include and what features to exclude. To decide this, you need to look at some good social networking websites such as orkut.com, facebook.com, badoo.com, etc and then list the features you want to include. I would also like you to refer this link http://www.techcrunch.com/2007/07/24/9-ways-to-build-your-own-social-network/
Once the features have been listed. Start with the database design then administrative section, then frontside web development.
