If your desiging a circuit from scratch - then chosing the right transistor is a bit of a black art.

That said, if you have an idea of the operating voltage and current draw you can narrow your search down.

Probably best to trawl the net for some circuits (similar to your idea) and copy that.

If your replacing a stuffed transistor - change it with the same or get your hands on a transistor comparison tables book to find an equiv.

Squib