Locks are like forks for BDSM - you pretty much need them at every sitting. So why not talk about them?
You need two different sized locks and several of them. The first kind of lock is a standard locker sized lock like this one here.
Note that it's about the size of what you used in your high school locker (if you had one) and it uses keys. I ALWAYS recommend locks with keys. The reason is simple - you are going to be using many of these locks, and probably different ones for each encounter. Remembering which combination goes with which lock when you're in the moment seems daunting. It's a lot easier to just have a key ring with all your keys on it so you can pop on or off a lock with ease. Stick with the keys. You'll go places.
It's also easier when you're getting to know a partner to give them keys rather than give them a combination. X has a single lock with a combination. I have literally never used it. It's just so much easier to use keys.
One thing you can do to make the keys even easier to use is to mark which key goes with which lock. Take a Sharpie marker and make a symbol on each set of key and lock so you can easily tell which key goes with which lock. Got black keys or locks? You can buy silver Sharpies that work great on black.
We use the big locks mostly for locking two things together. For example, if X is wearing two wrist cuffs and I want them linked together, rather than breaking out the rope and messing with knots (that can potentially slip or he could undo) I can just lock them together. I think you need at least two - one for wrists and one for ankles.
The second kind of lock you'll need is luggage sized locks. These are significantly smaller and are used for locking on collars and cuffs. Basically, most collars and cuffs have a small hole on the end of the buckle. You can slip a lock on there and even if your sub can get his hands to the buckle, he can't undo it because it's locked in place. I recommend getting a set (like the one I linked) so you can have one key meet all the locks. In my opinion, you need at least five - two for the wrists, two for the ankles, one for a collar.
Get yourself a makeup bag or other small zippered bag to hold all your locks and keys. This makes clean-up, storage, and travel easy. You can get makeup bags on Amazon or just at your local drug store.
It's a good idea to always have one place to put the keys. You could put them on a necklace and wear them. You could put them back in the bag. You could put them on the dresser. But make a habit out of putting them in the same place each and every time. I like to keep mine in our bag. That way, I always know exactly where they are; when we're done with play, I can let him out immediately or later, totally on my schedule. But it's a relaxed schedule instead of a frantic find-the-keys-in-the-sheets schedule.