"Hi" is normally an informal way of Greeting. The best way to greet a person is to say "Hello". It can be used both as formal and informal. Of course if you know the person and you have chatted with him or her before, then its fine to say "Hi" .

Try and use wherever possible the English language. Most of all, SPEAK on Language sites more than text chatting. Voice chat is essential and has to be done at least 1 hour in a day because that improves your verbal communication and jaw movement.It also helps you to get rid of your mother tongue.This worked for me to study a foreign language.Apart from speaking use the same technique for text chatting as well. if you require any help in English mail me or post me.

hi is used in a casual way whereas hello is used as both formal and casual. e.g. hello sir etc. its best to use hello as casual and formal.

According to me spoken English can only be improved by voice chatting.I'm studying a foreign language and i have improved it by voice chatting with native speakers.Your mouth muscles are toned to your mother tongue which in your case would be Mandarin i suppose. Try voice chatting more than text chatting.