It result from a combination of factors as mentioned in the various posts.
Certainly a steel string guitar you here a lot more ringing overtones that on a nylon string. Actually when your switch from nylon to steel string it can be quite disturbing for a few minutes until you get used to the "ringy-iness" again.
As you advance on classical guitar learning how to dampen the sound on strings you don't want to ring is an important skill too.
As a beginner you will have lots of unwanted sounds, eventually you will gain technique and control. Typical classical guitar lessons are based on a nylon string guitar which has a much wider fretboard which will influence how you play and how you are taught to play. So it would be a good idea to look into getting a "classical" guitar soon.