What's the deal with steroids?

I'm 16 and I want to gain some muscle by the summer time. I'm a bit on the scrawny side. However, I want to keep my..stuff. I hear steroids can make everything go smaller..which is something I don't want. Obviously. So do steroids really shrink your junk, and if not, which one, brand, or type would work the best in a short period of time?