One of the most straight forward things you can do to improve speaking, is to simply speak more slowly. Now I do not mean careful pronouncing every word, but instead focus on completing an entire thought before talking.

If you feel yourself beginning to ramble or becoming nervous. Simply pause, it might seem counter intuitive, but know that you are not pausing for a long as you think you are. And pausing is always better than 'umming' or 'aaahing'.

Also broaden your vocabulary, the best way is simply to read more books. So that next time when you as 'how to talk smart' you instead say 'how do I speak more cohesively'