@Daniel Hill Starr
I am being 'facetious' here ... Good Luck ... better you, than me ...
I 'cringe' when I have one ... Smith; Brown; Jones; etc.
It depends on what other information you have on the specific John Smith in whom you have an interest. Actually, I have sometimes found it easier to identify a person with a common name than one with a much rarer one. Often, you find cousins with the same rare name (first and last parts) who lived in the same area and were born around the same time, whereas sometimes there might only be one (say) John Smith to be found for miles around a certain location.
If performing a search on the FamilySearch website, be sure to enter as much detail as possible* relating to such an individual, then make use of the filter options on the left-hand side of the page to reduce the number of results that have been produced still further.
(* Except try avoiding entering too much - say birth, marriage and death details - in "one go": this will prove counterproductive. Just enter data in "different combinations" for best results.)
So, don't despair too much, even though (like Brett) I admit my initial reaction at finding a John Smith as a relative would be - oh, no! That said, if you don't have a clue where your John Smith came from you probably are in trouble!
I guest you enjoyed my answer.
I got two (x2) STRANGE E-mails.
The ACCEPTANCE being one ...
The OTHER was ACHIEVING, Level 1.
But, that link went nowhere.
That said ...
I got x2 MORE.
Both indicating I had ACHIEVED Level 2.
They at least went to my PROFILE.
This is such a common problem. Using other information such as occupation, address, children, etc. to sort the different John Smiths out is one way to sort these people out. I have had to use a spreadsheet to sort out various families. By following each John Smith throughout his life you can begin to see connections. I often do the family history for many John Smiths that with time I realize are not my ancestors, but I cannot know until I do the research. Slow and tedious, but the only accurate way to be sure. Good luck!