The FBI is looking for people with clean records, but they also know that everyone makes mistakes through their life.
We were all young and experimented, and maybe even drank a little too much, or smoked marijuana. Most people don't know this, but it is not enough to keep you out of the FBI, but what will keep you out is lying about it.
When it is time for you to fill out that background paperwork, tell the truth. You will be taking a polygraph test in the future and believe me, you won't be able to lie your way through it. Tell the truth!