I want to take optimum nutrition serious mass weight gainer. I am very lean and thin. I want to build my body by going to gym. My trainer suggested?

Keep it simple. A high protein diet will support muscle building. Fancy supplements are not needed, and can often be harmful.
Muscle builder. Point number one: Do not take anabolic steroids or growth hormone. Get 7-9 hours sleep - stimulates your natural GH Take 25-30 gms of whey protein with a banana and mix with water 30 minutes pre workout and a protein shake 30 -60 minutes after. Eat chicken, fish and lean meat 3 x daily. Fill up on whole grain quinoa and vegetables. Do not over train as that can cause muscle loss.