Ok, im 15 and i just starting working out. I work out from mondays-friday. I mostly work on upper body (machines). My friends told me that if i dont rest i'll tear a muscle and i wont gain any. Is this true? My plan was to work out on modnay-fridays and rest on Saturdays and Sundays. Is this enough rest?
You need to rest, your friend is correct.
Muscles work like this:
When you workout you push your muscles to the max, they get lots of little micro tears and this is why they hurt or feel sensitive, they then need to heal. Its during the healing process that the micro tears heal over and become stronger (and its also why its important to stretch-if the area is not stretched they don't tend to heal back so evenly and you end up with poor muscles and lack of flexibility.)
See, if you don't rest for long enough, they simply cannot heal, they either don't heal or they only partially heal, you wont gain any real stregnth and you could actually injure yourself as each time you workout, you weaken your muscles, thus working out with weaker muscles is a risk and can cause you an injury.
I'd say sat and sunday in theory would be good as rest days but if possible, can you not spread them out in the week? maybe something like resting on wednesday and sunday, just because this gives you a better workout, if you are pushing yourself to the max, you may find you get to friday or even thursday and cannot do as much- where as if your workouts are more spread out you may be able to get more done every day rather then only in the first few days.
Try it and see, if it doesn't work for yu then go back to the 5 days on, 2 days off routine but if your pushing youself hard its not going to go in your favor.