I will repeat what others have said but I will provide an example.
The example: I know an intelligent guy who scores perfectly on psychology and biology exams, but no one takes him seriously or considers him intelligent because he speaks like an idiot. They don’t believe him when he tells them his grades because they assume he could never score so highly. He does not read, ever, unless it is to study. He can’t even rhyme words. Ask him to find a rhyme for ‘clock’ and he will tell you ‘mop’.
So, yes, you have to read. But just as looking at great paintings won’t make you a great painter, reading won’t make you a good writer (i.e., a user-of-words, whether in your head or out loud). You will need to find your own method, but writing your thoughts down carefully, slowly, and regularly will lead, in an organic/natural way, to more articulate thoughts. You don’t need to write scholarly essays or anything; a journal will do.
Some people tend to think in images while others in words. Try to think in words more often, as an exercise. Also know that a mind works very quickly and sometimes we can come to conclusions, reach the endpoints of ideas without even having to articulate them mentally or vocally. It’s a wonderful thing to make mental leaps, sure. But sometimes an idea must be arrived at rather than teleported to. I have made it a habit to talk to myself aloud if no one is home and to to talk to myself silently (i.e. conduct interior mono/dialogues) when someone is home. It’s more an exercise than a lunacy thing, I promise. It has helped me to be more articulate both in conversation and at the keyboard.
And I wouldn’t worry much about vocabulary (though of course it helps). Syntactical and grammatical aspects are more important in the pursuit of clarity and articulation.
Just two pennies. It works for me.