what determines how many coins in my wallet are "staked"?
Staking basically takes the pieces of coins you have(Turn on coin control, then send and click inputs to see the pieces) and uses their quantity and age to choose who gets to mint the next POS block. Your coins are used in the block creation, they are split with interest added, and returned to you as staking. It then releases from stalking after the correct amount of blocks(60) following it have been met--this is to be sure the blockchain your wallet has, is the correct one and it's still valid.
Things can go wrong, currently a POW miner can be seconds before you with their calculation. Or due to the speed of ones connection, there might be another wallet generating a POS block at the same time, if all the other wallets see theirs first, you wait. These blocks show up as conflicted with a ? on the transaction. No worries, that chunk of coins will try to stake later.
Lets say you started by receiving 10,000 coins, first time they get staked you end up with lets say 2 coins interest, so two groups of 0 aged coins at 501 count each. Over time the groups of coins you have will increase, and it will take longer and longer for them to stake because of their small size. You can make them bigger again by sending them back to yourself at any time all collected together.
If you leave your wallet open it supports the network, and it will try to stake when the opportunity arises. Or you can leave it closed for a month, for example, and then start it back up with an even greater chance of staking. The interest is based on the coins age, so waiting doesn't lose any interest.
