The capacitor C is defined as the charge per voltage applied. C=Q/V Before you turn on the switch, if there is no charge on the capacitor, the voltage Vc=0; When you turn on the circuit, the current starts to flow into the capacitor (which in turn, increase the voltage of capacitor Vc=C*Q). You can check out our [url=http://www.phy.ntnu.edu.tw/ntnujava/viewtopic.php?t=48]RC circuit applet[/url]