Saying hello is not always as easy as you might think! We often change our expressions.
When you meet someone for the first time it is best to be polite and not too casual.
It's nice to meet you. - This is good.
'It's so nice to meet you.' - This is also good, but is familiar, so use it with someone who is close to you, such as your friend's mother.
* You can also change 'nice' to other adjectives such as 'lovely', 'good', 'great', 'wonderful' etc
For someone you work with:
How's it going? (Do not forget 'it') / Hi. How are you? - These are good for people who you feel are friends.
For people you do not know very well, a simple 'good morning' is best.
* Polite language is useful for people who we do not know well, or for people who are senior to us in position or age.
'Long time no see.' - This is useful, but every Japanese English speaker uses this expression! I suggest learning some different ones, such as;
'I haven't seen you for a while, how have you been?'
'How have you been?'
'It's been a long time, how are you?'
'How's life?'
'How are things?'
'It's been a while, are you well?'