Dude I am certainly no 'old timer' by anymeans. I really think that you must seriously consider what you are looking at doing. I myself am fully matured I believe and am approaching 23 but still guys have really advised me away from steroids in general. One thing that you must really get to grips with is the fact that these are not miracle drugs and unless you have the lifting experince and the diet to back that. You are not going to transform your body over a 12 week period. Getting the type of body that you may desire will take a long time probably up to years. One thing you can be garaunteed is that you will never find a person whom is completly happy with his or her's body. And this is why you find people whom do this as a lifestyle not just a decesion to go lift wight in a gym.
But with regards to what you are asking I would be carefull cus all those things that you are concerned about such as a underveloped body and so on. Can be part of the side affect ( They can inhibit further growth and the development of your body. Most definetly!!) Read through some of the red banners or stickys right at the top of the forum and these will give you a good indication of what side affects can occur. These posts are generally well written and have solid information posted by guys whom have a lot of experince.
What is your age??
What gym experience do you have??