Sometimes called 'Twang', it's a technique used in Musical Theatre to help the voice project, and can be thought of as a style of singing.
To limit or to reduce the amount of 'twang' in your voice, try singing a long note while opening and closing your nose and notice the difference in the tone as you do so.
You’ll probably hear and feel a nasally sound when you close your nose, like Mr. Bean!
What you’re after is no difference in tone when your nose is either open or closed.
Relax your jaw, throat and flattening your tongue, so it's not touching the hard palate.
By creating an 'open' space in your oral and nasal cavities, you should be able to reduce the nasal quality of your voice and adopt a more natural speaking voice.