When you make relative clauses, you pay attention to one thing:Merhaba,
I am trying to say this : I went to a friend of mine who is doctor.
My tries :
Doktor olan bir arkadaşıma gittim.
Doktor olduğu bir arkadaşıma gittim.
To remember this think of "-AN" like it's "the one"You introduced a relative after the phrase "a friend of mine". All right, and who is the subject of the relative clause? (Who is the doctor?) A friend of mine. So it's also the subject of the clause: You use -en/-an.