How do I do my job?

I've somehow found myself in a precarious position. I need you to tell me how to do my job.

I've been hired to build an infrastructure for a small facility. I haven't been given any details yet apart from the fact that they hired a bunch of vendors who half assed the job. I'm to design and build (or fix the vendors mistakes) a network, AD servers, and work stations.

Whelp, I've got 0 experience with any of this. I'm the entire IT department, and I don't even know where or how to begin. I labbed a lot in school, but purchasing stock equipment was never part of it. Based on my quick research, i don't think I can just simply purchase the minimum required Cisco equipment, configure and then deploy them. It seems more complicated than that. I also do not know what I am doing in regards to configuring a wireless network from the ground up or the AD server.

You're asking, "dude, why did you accept this position if you're not sure you can actually complete the duties?" Well, I've searched long for some sort of junior or "entry" level networking gig in which I can learn under the tutelage of someone more experienced than me and gain that much-needed professional experience. I have come to the conclusion either this doesn't exist, or it's extremely rare. And so this is my only option to get some experience.

Help me stop freaking out, please.