Communicate in clear and simple language
The use of plain and simple English is better than saying something that is incomprehensible or could have more than one meaning. If the people you are talking to have to ask for a clarification you are not a good communicator. Believe me, as a copywriter I know how important it is to be as simple and plain as possible.
You have used the word communicate. That means both verbal and written.
I would suggest reading newspapers (doesn’t everybody advise that?), but it is surprising that not many people heed this advice.
Write! This is a good way to increase vocabulary. Write about the subjects you would like to discuss. Once you have written, Edit! Think of ways you could say the same thing, in a better way. If it is targeted to a particular group of people (not everybody is interested in everything) then try using words that they would grasp faster.
Talk and start conversations. Talk as if someone is judging you on the number of times you have spoken (like a group discussion during an interview). Have an opinion on everything. Your reading should make you better informed and your writing should give you a better vocabulary. Your opinion could carry weight.
If and when people will seek you out just to talk to you or read what you have to say then you are doing good. Good luck!