Well, seeing that you used the word "toggle" in your question then you must have noticed the option for it when sending outputs to other entities. There are two easy ways I can think of ATM on how to do this.

1. You can send an output from the func_button that looks like this:
   my output named: OnPressed
   Targets entities named: light (or whatever)
   Via this input: Toggle

   ^That will give you a light that can be turned on or off with a single button. If you select the flag to tell the light to start off, then when you press the button, it will turn on. So, for instance, this method would work if you had several light switches in a room, and all gave them this output to the same light; they should function like a real set of light switches.

2. For entities that do not have a "toggle" input, you can use your func_button to perform these two (2) outputs to get the same effect of the previous method:
   My output named: OnIn
   Targets entities named: light (or whatever)
   Via this input: TurnOn

   -&-Another output that says:
   
   My output named: OnOut
   Targets entities named: light (or whatever)
   Via this input: TurnOff

   ^And that would give you the same effect as the "toggle" output. But this version really only works for one button, unless you don't mind your buttons having to be specified to an "on" or "off" state (just make sure the player realizes it, or it may frustrate them), rather than just a switch that will 'toggle' between the two modes.

Those are the easiest ways of using a single func_button, but if you think about it, you can create much more elaborate versions that can give you some flexibility by adding in criteria that may need to be met before a switch can work (from the light's end of the I/O system). BTW, I too did a quick forum search to no avail on this topic. I'm surprised it's never been posted before, this is a beginner?s topic, but one that eludes many people, (including myself once).