Google is your friend. There is loads of info on simple transistor audio amps.
A simple single transistor pre-amp is at http://hackaweek.com/hacks/?p=327
EDIT: If you do follow the hackaweek desig to build a preamp and LM386 amplifier them bear in mind you don't need resistor R1 shown in the circuit at the above link. You only need a transistor, 2 caps and 2 resistors to make a simple audio amp.
Try googling for '2 transistor 1 watt amplifier' as that will probably be better for your project.
OR you could buy a cheap audio amp IC for your project. Small ones of a couple of watts can be had for pennies. You can get higher powered stero amp IC's of 25Watts for £2-£4.