I have a 2N2222 transistor. How do I use it to amplify current?
Current into the base-emitter junction, higher current through the collector-emitter. 
We need to know the context of what you're doing or the application.
...Do you need linear amplification, or a "current boost" to drive a motor or something?
[u]Here is a current boost circuit[/u] for driving a motor or solenoid from an Arduino. You can optionally control higher voltage (higher than 5V) as well as higher current. If you have a non-inductive load, you can leave out the diode.
How do I use it to amplify current?
You do know that it doesn't actually make the current any bigger?
What it does is to cause an analog of the base current to flow in the collector / emitter. The current flowing here is supplied from the power supply. The original current us used in making the transistor work. A better name would have been valve but that name was already taken by the gaseous state device.
Amplify current or switch current? IE analog amplification or digital?
Grumpy_Mike:
A better name would have been valve but that name was already taken by the gaseous state device.
A valve (vacuum tube across the pond) exhibits transconductance. The new device was thought of as the dual of the valve so it exhibited transresistance hence the contraction of transresistor to transistor.
So what might be a good one word name for a field effect transistor which is a voltage rather than current controlled device?
Russell.
Besides just FET? These made from metal oxide semiconductor, or MOSFET?