You've probably heard that Korean (along with Japanese) is different than English due to confuscianism and the "social status" - mostly based on age - that comes along with that. Well what that means when it comes to learning how to say hi Korean is that you it's not quite as simple as just that - but it's not real difficult either!
The first way to say hi in Korean is the most formal way and personally I don't really recommend learning and using this one, since the next one I'll show you is more common - still formal formal, respectful, but a bit more casual - and there's really no point in learning all 3 since it can be a bit overwhelming and that energy could be put into learning something else more useful (just my opinion)!
So the first one is...
(The romanization is something like...Anyoung Hashimnikka? - I say "something like" because the system for romanizing Korean has changed enough times that there are more than too many different spellings for so many words that I don't really care which one is deemed "correct" at the time, especially since as I write this they are currently revising it again. And if you are planning on learning more than just how to say hi in Korean, I really don't recommend you rely romanization to learn how to say things - learning how to read Korean can seriously be done in a few hours if you really just sit down and focus on it).
The next, and probably most common way of saying hello or hi in Korean is...
(this one is a bit more casual than the first, but still quite formal - you would almost never greet someone older than you or in a higher "position" in anything less formal than this)
The third and also very common way to say hi in Korean is exactly the first two syllables of the two above examples...
(this is very commonly used, but only ever with someone you are familiar with and is the same age as you, or someone younger than you or in a lower "position" than you).
For the first two mentioned, you would usually throw in a bow (a slight bow is fine, at least you're trying) as you said them as well - it can take some getting used to, but really goes a long way when it comes to first impressions. Think of it as if you held out your hand to shake someone else's and they just just smiled and looked at you. Not really the most friendly gesture to start off with, even if it's a totally innocent mistake.
Anyways, hope that helps. I will be posting some vids helping with the pronunciation of those different forms of hello as well.
P.S. Oh yeah, I almost forgot! Hi in Korean on the telephone is 여보세요. Not real important to you just yet probably, but an interesting little tip (maybe).