I started gym 1 week ago..my trainer told me to take ..top gainer deca nutrition supplement..can i take it...is there any side effects..,am very thin?

Don't need them: since you are starting off early. Protein supplements are very expensive. You can get the SAME amino acids from normal food sources in dairy, eggs, fish, meat or tofu form. It tends to be MUCH cheaper than supplements, ounce for ounce! You should eat approximately 3000 calories per day, 25% coming from protein. IF you stop experiencing gains, you can add supplements later.