well this is easier with a diagram but i'll see if i can explain it
okay your source(in this case 5V) is tied to your collector. now there is also a resistor that goes from the source to the base and one from the base to ground, thats your voltage divider. you'll have to check to see what level that needs to be to turn the BJT on, i'd set the DC voltage to be just below that. the AC voltage will also be applied to the base and it will actual turn the BJT on(NOTE: remember to use a cap in series with the AC signal to take out all DC voltage). now you also need to know your Max current draw, once you have that you can find how much current can be applied to the base so you'll know how to set input impedance. the capacitor off the collector will run in series with the output resistance, this is used to again filter out any DC voltage. on the emitter side you're again going to have a voltage drop...the BJT will probably drop .7V the rest will be dropped across the emitter so choose a resistor size accordingly...this is the basic layout of the circuit, since i don't know what you're doing i can't give you any cap/resistor values but theres a start.
