"Hallo, wie haben seit du?" is an ungrammatical sentence without any discernable meaning.
Hallo - Hello
wie - how
haben - to have (infinitive)
seit - since
du - you
hi in german is hallo
The word for high is the same in German. High
Hi Jake Hallo Jake
Hallo, Hi
Hallo Feuerwehrmann
In German, "hi" and "hello" are the same, "Hallo". But you can also use the English "hi", since German is adapting a lot of new foreign words at the moment and hi has been used in German for many years already.
Hallo Blödmann
Hallo Guten Tag
Hallo. He. ('He' sounds just like 'Hey')
"Hallo Briana" or "He Briana" ('He' sounds just like 'Hey')
Hallo, hi, Servus, Grüß Dich
Hallo!