How do I make a command block output a Redstone signal when it is successful?

You'll need to use a redstone comparator and probably a repeater as well.

A comparator will signal in the same fashion as it does for other non-inventory non-selector blocks; when the command block activates, it will be detectable by the comparator and will only produce a signal strength of redstone equal to the number of successful activations. Most commands only activate 1 time, so a repeater would be useful here.