I am a 20 year old guy.however,my body development has not taken place correctly.my weight is just 45 kg.beard has also not developed properly.I am having only a moustache.Due to this ,I remain worried a lot.What should I do?Should I use some steroids?Can an expert please help me out?

First thing, talk to your doctor. Even if your hormones are out of whack and steroids are an option (and that's not definite, it's quite likely to be due to very low body weight), then only your doctor can legally prescribe them and give you the correct amount.