"Hi" and "Hello" mean the same thing, but "Hi" is more casual.

Use "Hello" when you meet someone for the first time in a formal situation:

Hello, pleased to meet you. My name is Arjun.

When you meet someone for the first time in a casual situation, like when a friend introduces you to someone, "Hi" sounds more friendly.

Hi Sara, nice to meet you.

When you greet someone who you already know, "Hi" is more common. "Hello" is OK too, but it seems more formal. If you always say "Hello" to someone it might seem like you don't feel comfortable with that person. So use "Hi" most of the time with friends, coworkers, your boss, your neighbors, and other people who you know.