I have a terrible speaking voice. How can I improve this?

My voice is weak. I mumble. I can't speak with very good tonality. I utterly fail at doing impressions. I usually can't bring myself to speak with any confidence. I can't enunciate and articulate words very well. I don't even really like the tone of my voice.

Can I greatly improve these things with practice, or am I doomed to have an average voice at best? How can I improve these things?