The first rule of speaking English is to speak clearly, concisely and use simple vocabulary. KISS – keep it short and simple.
Remember you probably won’t just speak to native speakers. There are roughly 380 million native speakers, but as many as a billion people speak it as a second language. So it’s a good idea to avoid idioms and slang (I always say learn it, but don’t use it). It might sound clever to say “You’re barking up the wrong tree,” but if you misuse it or if the other person doesn’t understand you, you’ll only look silly when you try to explain what you meant to say, or what it actually means.
There’s also a saying in English “Have you swallowed a dictionary?” It is applicable to anyone who uses long, complicated words when a shorter word will do. Short sentences are just as good (if not better) than long explanations. The value in what you have to say is what you say, not how clever you look or sound when you say it.
Accents don’t matter, but pronunciation is important