hi
Im 18 m and ive been doing body building for 4 months, the reason why i started was to get rid of the fat in my chest. but since my arms are thin the coach of the gym said that i shouldnt do cardio and the fatty breast will go by chest-exercises like the benchpress.

but i dont see any improvement(my chest is only a little harder, but the fat remains)

Some people advice to make benchpresses of low weight with high rep, and some say high weight-high rep) and since I cant do cardio(coz i'd be very thin) what do you suggest? is there a special exercise to burn the fat of chest? thx again

You can't spot reduce, you can't only burn fat from your chest.

To loose fat, you MUST do cardio. Doing benchpresses and other chest exercises may help since your pecks will get harder and bigger, but to loose that layer of fat, you must do cardio.