Being able to speak your thoughts requires a combination of knowing how to generally communicate through language and knowing how to specifically communicate with the person who is listening.
There are a few key elements that contribute to successful communication on a general scale: structure, organization, and clarity.
Structure: Know what kind of speech you want to have - do you want to give a monologue on your thoughts, where you are explaining your thoughts to a listener, or do you want to have a conversation, where you invite a person to listen and to respond to your thoughts, as well as share their own?
Organization: Be sure that you know how the different elements of your speech connect to one another - if your thoughts began with a question, start by introducing that question, retrace your discovery process, then share your conclusion. If your idea includes a contradiction, introduce the main idea first, then the contradiction.
Clarity: Organization contributes a lot to clarity, but so does word choice (also known as diction). Avoid bland words like "good", "thing", and "say". Also avoid filler words such as "like" or "um". The English language has more than one million words - there is definitely one in the dictionary that can more accurately tell your listener exactly what you mean. Choosing the right word first avoids confusion and questions later.
When it comes to successful communication on a specific basis, consider with whom you are communicating.
Relationship: Your relationship with this person will affect how you choose to communicate with him/her. It will influence your word choice, the type of communication you have, and even your willingness to have a conversation vs. give a speech.
Personality: Knowing your listener's background, likes/dislikes, and how she/he thinks can help you communicate. A listener who has trouble paying attention, has strong emotions, and has never attended college will be very different from a listener who fixates his/her attention on one thing, is logical and unemotional, and has a PhD.
Hope this helps!