Research
There are so many resources on the internet. Tumblr is a huge one, as a matter of fact. Tumblr, Pinterest, and even just Googling things give you a tremendous amount of resources at the edge of your fingertips. From various voices and accents to describing skin color and other physical descriptions, various cultures, and religions, and so much more. Just know what you're looking for and don't be afraid to search for it.
Talk about it
I'm sure you all have friends that come from different backgrounds than you. Study their mannerisms, ask them about their culture. Don't be afraid to want to know more, to understand more about them.
Read
You've all heard of books, right? Look up books that center a protagonist that's similar to yours and learn how that author pulled it off. Brush up on your history, as well, depending on what time period you're writing in. Remember, it's one thing to add in diverse characters, but it's another thing to add them in and actually portray them correctly.
Diverse characters can include various:
Races
Ethnicities
Classes
LGBTQ
Medical disabilities
Physical disabilities
Mental disabilities
And I'm sure there's a lot more that I'm missing. (if I'm missing anything, let me know)
