DosBox Wiki is a good place to start.
The config file explanation presented there is pretty extensive.
If a game works choppy under DosBox, use CTRL+F12 to increase the cyclecount it is running at until you reach a playable value. If your machine cannot handle that high cycles, go to DosBox.conf and change "core=normal" to "core=dynamic".
This will allow for approximately double the usual cycle count, but may cause occasional crashes.