When you are speaking over the phone, you would say "Hi, it's [name]" When you are writing an email or a letter, the name is usually left to the end after the closing (i.e. Sincerely, [name]) or in some cases, you may add your name at the start of the letter such as "Hi ____, this is [name]" Through text, you usually do not say "Hi, it's [name]" unless if the person did not have your number before and did not know who it was. Otherwise, you would just assume the person knows who you are and you do not need to say it is you. When you are meeting someone for the first time, then you would say "Hi, I'm [name]"