Hey guys I need help, I have a problem with my voice.

I feel as though its quite mumbley and I often have people ask me to repeat stuff more than twice. I also dont pronounce words very well e.g instead of 'then' I say 'den'

So I guess I need to work on pronunciation

Do you guys have any resources I can use or could suggest some exercises? I was thinking of devoting 15minutes each day to tongue twisters etc but I'm not sure if this would help much.

Thanks