The word 'hi' is an informal abbreviation of hello, and so has no direct translation in Swahili. Instead, you could use something like "hujambo" or "jambo" - which is the equivalent of saying "hello".
However, 'Hi' in itself may be common enough in Swahili speaking nations, due to the prevalence of English-speaking films and TV shows in all parts of the world.
Greetings are an important part of Swahili linguistic culture and if you are conversing with a native speaker of Swahili, you should remember that opening a conversation involves more than saying hello. That said, there are some short useful phrases you can use when you want to greet someone.
How to Greet Someone in Swahili
Hello - hujambo or jambo
Good morning - asubuhi njema or habari za asubuhi?
Good afternoon - habari za mchana?
Good evening - jioni nzuri or habari za usiku?
How are you? - jinsi ni wewe
To learn more about greetings in Swahili, watch this film: