If you knew you were holding drugs, you would be as guilty of the drug sale as your friend. It’s called "aiding and abetting" a felony. Your only hope of getting off on this charge would be if your attorney could create reasonable doubt in the minds of the jury as to whether you really knew you were holding drugs for your friend. The exact situation you were in, and the circumstances of how you were caught, will affect the outcome of this type of a case.
Determining your accomplice status
If you were standing on the street corner and a police officer witnessed you pull a baggie (of, say, methamphetamine) from your pocket and hand it to your friend, who then sold the drugs to a third party, you may have a difficult time proving that you had no idea that the baggie contained drugs. This is due to your surroundings and behavior at the time. You may be seen by the jury to have been acting as suspiciously as the friend who actually completed the drug sale.
On the other hand, if you were out with your friend and the friend asked you to hold his backpack, and the arrest for sales occurred after you returned the backpack, it might be easier to convince a jury that you had no idea the backpack contained drugs but were simply doing your friend a favor and holding his bag for a minute. Either way, the bottom line is that you need to hire the best lawyer you can afford. Drug sales, and aiding and abetting a felony, are serious crimes. This is more serious than possession of a controlled substance, and could carry with it actual jail or prison time, plus a permanent criminal record that will affect your employment opportunities and many other aspects of your life for years to come.
Would a "friend" put you in that sort of situation?
You should be aware that your “friend” intentionally put you in danger of going to jail, most likely to decrease HIS chances of going to jail. This is a common strategy for drug dealers that, unfortunately, they sometimes get away with. You might consider offering, through your lawyer, to give the prosecution information that will lead to the conviction of your "friend" in order to spare yourself. ONLY do this through your lawyer. This may sound undesirable, but keep in mind that your friend wasn't concerned about putting you in danger of losing your freedom, and may very well decide to say that YOU were HIS supplier, in order to get himself a better deal with the prosecution. Talk to a good criminal lawyer immediately to find out what all your options may be.