Amin chord has only three notes, A C E. Amin7 has one extra note. Yes, you guessed it,Yeah I understand. It's hard, I don't know exactly how to play like that.
On a side note; What's the difference between a chord such as A minor, and a chord such as A minor 7th?
the extra note is the 7th note of the A minor dorian mode, which is the note G, so you have
A C E G for Amin7.