In general, if it is the action (친구를 만나요) which is to be emphasized, the 2nd one (나는 친구를 만나요) is used.
On the other hand, if it is the subject (나) which is to be emphasized, the 1st one (내가 친구를 만나요) is used.
However, if there is a comparison between subjects, e.g.,
나는 친구를 만나는데 여동생은 집에서 공부해요.
(Though I meet my friend, my younger sister studies at home.)
"~은/는" is to be used.
좋은 하루를 지내세요.／I wish you a good day.