Once Ethereum makes the switch to proof of stake how will one choose to stake Ether? Will they have to use a special program or will Ether not in use be considered staked and be used to gain more Ether?
The Ethereum Launchpad has the instructions for staking:
The launchpad informs you of the requirements, including running one of the Lighthouse, Nimbus, Prysm, Teku clients.
The ETH Staker community is highly recommended. They are very knowledgable, but their first value is "Welcoming First, Knowledgeable Second":