As Lee says, at WDW you pay to rent a locker for the day. You get a key and that is yours for the day. I think the cost includes a refundable deposit.
As far as 'which set of lockers to use for MK', there is only one set. They are inside the main gates, underneath the train station.
At the Universal parks, you have two types of lockers - those you can rent for the full day (like at WDW), which are located at the front of the park, and those you use for rides. Where a ride does not allow bags on, the lockers are free for the period of time you ride (the free period is based on the current wait time for the ride plus a bit of leeway). For those, you use a locker station, the instructions are pretty straight-forward. For water rides at Universal parks, if you choose to rent a locker, you will have to pay. You will need $$ notes or coins for these.