If your gun is a commercially made gun you do not have to "set" the rear trigger to fire the gun.
Just cock the hammer to full cock and pull the front trigger. That will drop the hammer (or cock if it is a Flintlock).
If you want a very light trigger pull, you can do like tv racing fan said and pull the rear trigger to "set" the triggers. The rear trigger by the way is actually just a way of cocking a heavy spring inside the trigger assembly.
With the rear trigger "set", a very light touch on the front trigger will release the rear trigger which will cause the cocked hammer to fall.
All is not free however.
The general design of many set triggers makes firing the gun without "setting" the rear trigger and using the front trigger only a fairly heavy trigger pull.
In other words, firing the gun using just the front trigger often has a long heavy trigger pull.
A word about "Half-Cock":
There is a special little device inside your lock that allows it to work with these Set Triggers.
It is called a "fly".
The thing you need to know about it is that once the hammer is fully cocked you cannot just pull the front trigger and lower the hammer directly to the half cock position.
If you do, the hammer may seem to stop at half-cock but it is not really at half cock.
The internal parts of the lock may "hang up" on the fly and any bump can cause it to fall to the fired position.
The way to get around this is with the hammer fully cocked and your thumb holding it, pull the front trigger and allow the hammer to gently move down to
almost the fired position.
Then, raise the hammer until you hear the distinctive sound of the lock going to the "half-cock" position. In this position the front trigger should not be able to release the hammer.
Another thing you need to know about your double set triggers.
With the hammer in the fired position you may set and fire the triggers as many times as you wish to.
This is handy for practicing firing the triggers from a "set" position and giving you a good feel for using the gun.
The thing you must never do is to set the hammer in the half-cock position and then set and release the triggers.
Doing this may break the locks sear or tumbler or both.
I hope this helps.
Have fun.