Do I have to enter USA within first 6 months after visa issued date . I am on tourist visa ?
You must use the visa during its period of validity. The first use of the visa can be at any point during the period of validity.
If you wait a long time before you use the visa, it's possible that the immigration officer might ask you why you waited so long to use the visa. Any truthful answer you give will be satisfactory; the officer is just trying to evaluate whether you are a genuine visitor, and asking about the facts surrounding your visit is the main technique they use to evaluate you.