There's no such thing as a "master key" in modern lock technology. There
are locks which have been set up to respond to master keying systems,
and one (almost any one!) of the possible keys for that set of locks
will be designated as the master and all the locks will respond to it as
well as to their individual keys.
(BTW: Yes, I know I'm oversimplifying slightly; there are sectional
keyways and secondary mechanisms and so on. And yes, there are a few
warded locks still out there. But all of that's far beyond this
questioner's level of understanding, and I suspect it's beyond his level
of interest since it doesn't change the basic answer of "no, there's no
easy way to get a master key if you aren't supposed to have one, and for
most locks there is no master.")
If this is for a pin type lock (Yale), if you have a key to a lock if pull
the pins out of the lock you can see the pins used for your key, if there is
only one master spacer in each pin stack you can make your master key.
Most large systems use more than one master sapcer in each stack making a
large nunmber of keys to try.
Without the right tools the above could be destructive and you could be
Sure. Here goes. First, you choose the code of the key you want to be your
master key. Then you buy several locks of the same brand. You generate a
master key bitting chart, and then key the locks to match your designated
master, and each lock on a different change key.
Most keys can be a master key, you just have to make locks that work on it.