A resource that most writers used to have (before everyone started looking up everything online -- which I still think should be an adjunct not the only source, because the information online can be very unreliable) is any What to Name the Baby type book.
I have about five. One is all Celtic names, and the others are more general. You can get them at any chain bookstore. Most will give you the origins of the name, the meaning, and every conceivable variant.
For a medieval English type setting, you can't go wrong with plain old common names like Richard, Edward, Robert, John. They wouldn't have gone in much for names like Alexander, I'm afraid. Probably too foreign. If you look at any of the lists or inventories for the period, you'll see that every village had several Johns, several Williams, several Richards, and so on. Hence the need for nicknames and one of the reasons that surnames began to develop. So that you could have Robert Fletcher, who makes arrows, and Robert Miller, who grinds the wheat, as well as assorted Robs and Robins identified by what they do or where they live or what they look like. You could just about bet that more than half the men in the town or village would be John or Jack.
Although to keep your readers from going absolutely crazy keeping track of everyone you might want to use a little artistic license and introduce a little more variety than that.