James 1:14-15 - ...each one is tempted when, by his own evil desire, he is dragged away and enticed. Then, after desire has conceived, it gives birth to sin; and sin, when it is full-grown, gives birth to death (NIV)
So we sin when we give into temptation. Sin is acting outside the will of God, and of course as Christians we get tempted to live outside God's will on a frequent basis. I'm not going to list the methods through which we get tempted - but I'm sure the media would be right up there. Regardless of how we get tempted, I consider that sin happens at the point when we make a conscious decision to step out of God's will.
In Romans, Paul says that he finds himself doing things he doesn't want to do. But he also says that he is no longer a slave to sin, but rather a slave to righteousness. In Romans 8, having just written about having a sinful nature which is still struggling with sin, he writes that we do not live according to the sinful nature, but according to the Spirit.
So live that way! Don't sin. Live according to the Spirit instead.