How do I use/add first name (or forename) in citation?
It's a little tricky trying to expand someone's name, since you might inadvertently arrive at a form they never use (for example, some people have never been called by their first names, but have always used their middle names instead, and may prefer to be cited as "F. Middle Last" rather than "First Last", "First M. Last", or "First Middle Last").  Some people also prefer to use initials when publishing, for example out of fear of prejudice or bias.

So if the published paper uses initials, then it is safest to retain them when citing it, unless you are really confident that your expanded version of the name won't confuse readers or offend the author.

On the other hand, if the published paper gives a first name, then you have your answer.  Other databases may also give first names (for example, MathSciNet or the Mathematics Genealogy Project), but which databases are relevant depends on the field.