There are many people with D. in their titles like Monkey D. Luffy, Gol D. Roger, though we know they call it "Will of D" but what exactly is it. And how they acquire it by birth.
Not a manga reader but based on what I was able to search on the Internet, it is still a mystery and the whole truth isn't quite revealed yet.
From what I understand, it is still not known as to how it is acquired or if it can be acquired by someone consciously. It is, however, known that:
...all share a similar faith in their own (and others') dreams and/or destiny. When a "D." appears, the general consensus is that they are going to stir things up on a global scale, for better or for worse.
The second thing about the D is the "Inherited Will" which passes down generation from generation according to Roger and Whitebeard. Even if the life of the person is extinguished others will take up their will.
Also from the wiki, only Gol D. Roger from among those who carry the 'D' in their name was able to know the whole meaning and truth behind. If I understand correctly, what he knew has not yet been revealed in the manga. There are a number of theories about this in the Internet which you can look up if you are interested but bear in mind that these are just theories so there is no definite conclusion as to what it is exactly.