Been trolling through the forums picking up lots of good info, so time to return the favor.
The OP's question was answered, but if you are actually trying to use a very small mechanical relay (like a "reed relay"), you should be able to drive the relay directly with the Arduino pin, possibly with a series resistor, and the diode.
You need to look to see what the current requirement is, or calculate it from the voltage and coil resistance. The Arduino can drive 40 ma. Lots of the small relays you can find can be driven by the pin directly: you don't need the transistor.
You DO need the diode. The part of the relay that moves the contact is a coil, and when the drive is shut off, the field around the coil collapses, creating a voltage spike with a polarity opposite what was driving the coil. This is bad news for the Arduino. The diode effectively shorts this back EMF. For a small relay, a small diode like a 1N4148 will be fine.