How to make sure people understand what you're saying
The fear of not being understood when speaking English or writing an email or a message is almost universal. Practically everyone who studies a foreign language is terrified of forgetting an auxiliary verb in the middle of a sentence, of not remembering the right term, or of mispronouncing a word.
Obviously, it’s quite important to be as accurate as you can, especially when we’re talking about a big presentation, a corporate event, or writing a contract. On those occasions, preparation is not only important; it is key.

Learn key sentences in the beginning
If your fear derives from the fact that you still don’t feel ready to think in English and come up with your own sentences because you lack vocabulary, learning key expressions in the beginning (for example, to talk about yourself, order a meal or even ask for information) helps you feel more secure about your own English skills.
Practice these sentences as often as you can so you can feel more confident and willing to speak from the very beginning.
The English Conversation Practice / Speak English Conversation app has lots of dialogues with sentences and exercises for you to roleplay.
Create scripts
Prepare scripts of what you’re going to have to say or want to say. Are you traveling any time soon? Prepare conversations you might have with the Immigration officer, at the airport, at the hotel lobby, in restaurants, stores, in an Uber… Practice with the recorder on your phone until you feel confident about your pronunciation.
Do you have to take phone calls from foreigners? Prepares scripts thinking about how to take messages, put people through, reschedule meetings, etc. Practice as much as you can; when these situations do happen, you’ll be prepared for most of them and won’t need to rely on your memory – which is likely to fail if you haven’t used those words/expressions in a long time.
Speak more slowly
Many students watch movies and videos or listen to music and have the false impression that in order to speak English well one has to speak fast. Many people want to copy what native speakers do and end up having a hard time pronouncing certain words, which happens when you don’t have enough practice: you simply don’t pronounce certain sounds or even entire words, and that can hinder communication because the other person won’t understand what you say.
Keeping your nerves in check and speaking more slowly (a natural speaking rhythm) is a great way to start becoming more confident. Being understood is your number one priority, not sounding like a native (whatever that means).
For all of the three techniques mentioned above, your cell phone recorder is invaluable. Practice pronunciation, intonation, and rhythm if you want to speak English well.
Building up the courage to speak doesn’t happen overnight. But with those three guided practice ideas, it’ll happen much faster.