If you want to do this on your own, you may want to investigate lessons geared toward singing voices. The exercises one uses for strengthening the singing voice will also develop the diaphragm and laryngeal muscles to improve control and projection of one’s speaking voice. I am often complimented on my speaking voice, and the follow-up question is commonly, “Do you sing?” Similarly, I’ve noticed that many of the people who speak well are all singers. I did sing from preschool age in church choirs and through school in choruses and a capella groups. My Bachelor’s degree is in Speech Therapy, and my Master’s is in Audiology, so I’ve spoken to numerous hearing impaired people on a daily basis as my vocation for the past 20+ years.
Some yoga practices work on breathing and some of those exercises employ vocalization, which might also be helpful. I’ve taken yoga classes but never studied the terminology to advise you which forms of yoga do that, but I sure that can easily be searched on the web.
I am not a good resource for English improvement. It was always one of my least favorite subjects, and I never did well with my attempts to learn other languages, either. Most people develop a sense of language from exposure (and immersion is even better), so reading and listening to examples of what you’d like to improve are the first step. Could you expand on that request a bit more? What is your native language? Which variant of English are you hoping to improve? British and American English are quite different, and some websites and computer programs have separate British and American versions.