Welcome to the forum, cutenlover.
It's customary to put the other person first.
We play tennis on a court.
We use 'the grass court' if we mean a particular grass court and 'a grass court', or 'on grass courts' if any grass court will do.
"My friend and I like to play tennis on a/the grass court."
"My friend and I like to play tennis on grass courts."
Student or Learner