You don't say what it is about your voice that you'd like to improve. So the first thing to think about is -- what don't you like?
If you want better projection ground yourself before you speak. Make sure you are standing solidly on both feet and draw your breath up from the ground and out your mouth. Think of your body as a bellows.The greater/deeper the breath, the more powerful the sound.
If you are concerned with sounding monotone, take a song you know well, and speak-sing it. It's not about hitting the notes, but putting "musicality" into your voice by using a varying your pitch and speed.
Speak more slowly than you would conversationally, to give people time what you're saying. Pause.
Finally, if you have an unusual voice, consider the possibility that it could be a great differentiator. Arnold Schwarzenegger's accent...Victoria Jackson's high pitch...Danny DeVito's Brooklynese all give them a kind of memorability they wouldn't have if they sounded like everyone else.
I teach Public Speaking skills. I'd be happy to discuss this with you further.
Good luck!