Hakuna has got it spot on, go to the gym, and with each exercise you do, use correct posture and think about it constantly. Good form will correct good posture as the muscles will develop in that way and it will naturally correct.
Practice talking loudly at home, just project your voice. Don't overthink what you are going to say to the point where you mumble. If you have something to say, think, is this appropriate, and then just say it. Dont pause and stumble over your words. It will come more naturally over time just keep practicing.
Don't cross your arms, hover around people, just be confident and walk like you have a purpose and when you want to speak to someone just project strength so they can't ignore you.