Dear friend __ I need your help . Fill in the blank with suitable punctuation.

Dear friend __ I need your help . Fill in the blank with suitable punctuation.

Explanation
 Dear friend, I need your help.

The suitable punctuation to fill in the blank is a comma.
A comma is used to separate the salutation ("Dear friend") from the main body of the sentence ("I need your help").
It helps to create a clear and grammatically correct sentence structure.