Tips on becoming a Voice impressionist?

All my life, I’ve wanted to be a comedian. With stress, colleges and school work however, I’ve kind of put my comedy on the back burner.
Now, I really want to get into Comedy again, and I REALLY want to learn how to do voice/celebrity impressions.

Does anyone with even slight experience have any tips to offer? How do you change your voice?
Is it something ALL impressionists train themselves or get trained for? Or is it something you’re able to do from the day you can talk?