'a' or 'an'?

I have a doubt which may not be relevant to CA/CWA/CS but it would be helpful for me as a student if you can clarify this:

Which article i.e., 'a' or 'an' should be used before the worlds starting with the letter 'h'?

For example, refer the following:

The Blind CA Student has to take an under-graduate as ___ (a/an) helper to the Examination hall for writing the exam.

In the above sentence, which article (a/an) should be used for filling the blank?