can't get pregnant

i had my first son when i was 21. and he is now 3 years old. iv been wanting to have another baby but i'm not having any luck. this is making me depressed and tonbecome concerned . i'm afraid if i go to the doctors they will tell me that something is wrong with me. i'm very happy to have the one that i have . but i would love for him to have a baby brother or sister . it seems like everyone around me is having babies left and right and i'm over here trying and not succeeding :(