Was it one of the plastic hydrometers? If so, did your friend show you how to use it?
They're kind of tricky to get a semi-accurate reading because you'll get bubbles and it will be way off.
I always gently smack mine back and forth on the tank while it's still submerged until there's no visible bubbles (on the front or back of the pointer). Check the water several times to make sure your reading coincides with all the other readings. Sometimes the arrow will stick also.
If possible buy a couple of them. Make sure the readings coincide with each other really close. If one is just WAY off, or reads a different number every time that one is probably bad, so use the other one.
I bought one the other day where the arrow kept sticking and when I tried to measure my salinity it would stick at like 1.006. I even tried banging it really hard on stuff and it wouldn't unstick, so you can get some bad ones sometimes. If there's extremely innaccurate or sticky the store should take them back.