Truthfully there are a lot of ways to greet someone in English. We usually start by saying hello or a version of hello when it is someone we are meeting for the first time. For a friend any of these are applicable, but some are used more depending on the person's personal preference.
1. Hello - Standard greeting. "Hello!" "Hi!"
2. How do you do? - Greeting with more implication. Usually you want an answer. "How do you do?" "I'm doing well."
3. How's life treating you? - same as 2 but sounds more polite. "How's life treating you?" "It's treating me well!"
4. What's up? - This one is the same as 2 but more informal. There is a slang to this phrase which is just Sup. "What's up?" "Not much"
I would say the most frequent greeting for a friend is What's up?, or Hey!
Some other greetings: How's it hanging?, How's it been?, Long time no see, Yo, Hi, Hey, How you doing?
Hope I could help a bit!