Reserve?
 

So, I have invseted in Reserve skill, It should allow me to keep a reserved troop that doesn't participate in combat...
The question is: How am I supposed to use it?

Just click on the reserve troop and then click on the slot where you want it.
You can replenish an existing stack or you can make entirely new stack providing you don't have any of those units in your troop.

If you mean how to buy units for the reserve - it is just bad design - you must buy units for your normal stack, then click on the reserve and transfer your normal units to the reserve. The good thing is you can do it as many times as you want.

You use it to store either extras of one unit you already have, to replenish losses, or to store a troop that you may want to use in some battles but not others (ie. red or black dragons (they own against undead, thorns, ents etc., but when you are fighting demons they should remain in reserve))

Wait wait, where am I supposed to click again?
Where can I find that "rerserve troop" button? (or by "reserve troop" you mean button, right?)
I just can't find it :C

that is the arrow showing up that is over the troop list ( where u can see theyre pics )

Here's the button:
http://img73.imageshack.us/img73/378...1125001ui1.jpg

Aaah
I can see it now
I thought it's somewhere in Hero panel
Thanks for help ^^

It is actually available in the hero panel. There are 2 slots which are [N/A] in the beginning next to the active army slots. Once you get Reserve 1, one of the slot will be available.