Informal:
* "Hey buddy, want to grab a burger after work?"
* "My buddy helped me move all my furniture."
* "Don't worry, I got your back, buddy."
* "That's a good idea, buddy."
More formal:
* "My buddy from college is visiting this weekend."
* "I've known my buddy since we were kids."
* "The two buddies decided to start a business together."
As a term of endearment:
* "Don't worry, buddy, everything will be alright."
* "Good job, buddy! You did it!"
Remember that "buddy" is a casual word, so it's best to use it in informal settings.