Usually when I talk and write a mail I am doubtful to use " required" or " need" for the situation. please help me clear understanding of the place and situation to use it properly. Thanks in advance.
Need is clearly more intensive. If you need something, it's pretty crucial. On the other hand, if you require something, it could be just helpful.
To quote OALD,
to require something/somebody because they are essential or very important, not just because you would like to have them
So, food is our need and not requirement.
Nevertheless, you are writing an email where both can be interchanged in most of the cases.
Note: I may tell you specifically if you tell me where exactly you find it difficult.