To protect a chest, right click on it (do not sneak) while holding any sign
This will place the sign on the chest and automatically protect it, so that only you may open it.
If this doesn't work, you can manually lock it by sneaking and right clicking on the chest with the sign.
Then to protect it, all you need to do is write
[Private]
on the top line, and when you exit
out of the sign, it should automatically write your name on the second line and lock the chest
/lockette
tells you how to add users to a sign. You can also label signs like this, just
don't put a player's username on the sign by itself or else they can also access that chest
Examples:
/lockette 3 TheSenseiChiken -- /lockette 3 (Ores)

The first example would add TheSenseiChiken to the chest, so he would be able to open it. The second would write (Ores) on
the third line of the chest, just as a label (You can't put special characters like ( [ { , . - = + in a username, so if you
include those on the line, it would be impossible for a user to access the chest, even if their name was, for example, "Ores"