When I am writing email, if I use just Hello how will this affect to the reader?
It depends who you are writing to - if your topic is informal, such as an assignment for school where you have been asked to approach your peer group for help with their responses in filling out a questionnaire for market research for example, then it would be fine to address your email contacts with a 'Hello.' However, it would still be nice to address each one individually by name as their response to a 'blanket email posting' might be negative or even hostile. People are touched that you made the effort to find out/use their name in a personal approach.
If, on the other hand, your mission is very serious such as a request for internship or work experience, then of cours e you will want to make the best possible impression you can. Do not be overly personal/familiar here as the recipient is a professional and may find the approach off-hand or even arrogant. It is better to use concise appropriate grammar/letter writing forms - just a little more concise than a letter without leaving out any info.
Your question is a bit difficult to answer, since all people are different and you know people in different levels of intimacy. If they are close friends, I think a "hello" is just fine. However, I think when it comes to people that you do not know, then something more formal might be a good idea, such as "Dear Sir/Ma'am." Another alternative would be "To whom it may concern." The point is as follows: if you are not familiar with the person, be more formal. This will give a better impression. If you know the person very well, almost any address would be appropriate.