In settings->options you can set Reservebalance (it's the amount you don't want to stack)
In settings->options->display you can tick coin control, so you can control what coin you send (don't send old coins)
In settings you can unlock wallet for stacking only if you encrypted it.

If you have coins older than 12 hours, then you'll start to stack and you can see current staking status hovering the lightning icon.
But beware this is the "expected" time, it can be shorter or longer, as in POW blocks.

For starters go hover the lightning icon and read what it says .