Half the battle in convincing someone that you’re credible and professional is to believe it yourself first. If you don’t think you’re worthwhile or deserving of respect, it will be hard to persuade others of it. However the next problem confronting you if English is your second language is to find the correct words. I like to pass around the following resource to anyone and everyone. It’s presented as a resume-enhancer but you can also use these words in cover letters, LinkedIn About sections, professional biographies on websites, and conversation. And the great thing about this link is that the author had divided the vocabulary up into common helpful categories:
https://www.themuse.com/advice/185-powerful-verbs-that-will-make-your-resume-awesome