Hi!
But try to say it with some intonation. Hi! Hi! Hi!
But try not to make it short and low like hello.
Give some intonation to it. Hello! Hello, hello!
This gives a feeling to the other person like you are glad to see them.
Now for very informal or more informal we could say:
Hey!
I see somebody I know I can say hey!
It's very common to say that also give it that intonation.
Somebody that I'm very close friends with and younger people use this is:
You know it's very friendly it's very positive but it's only for somebody for you very good friends with and mostly younger people use that.
After you say Hello or Hi and you want to make some small talk.
How's it going? Notice how I reduce it:
Notice the intonation that I use. Try to use that intonation or something similar because it shows that you're interested in the other person and you have positive feelings towards the other person.
If the person that you say hello to is friend or somebody who you know and you're comfortable with you can use these phrases:
Show some enthusiasm whit these expressions but remember!
These are casual expressions and you should only use them with people who you are very friendly with.
A more formal way to greet somebody is to say:
Depending on the time of day but remember this is formal if you don't know the person.
Now to make it casual some people will say morning.
Morning meaning good morning or
Evening not many people say afternoon for evening.
Most people will just say morning, and then I'll just say higher hello to you in the afternoon.