This page will help you get started with VRChat. You'll be up and running in a jiffy!
If you're completely new to Unity it might be a good idea to learn a bit about the Unity interface. The first 6 videos in the Unity essentials series is a great way to do so.
First of all, make sure you've got the correct Unity (5.6.3p1) installed for the current VRChat SDK and download the latest VRChat SDK
Either create or open up existing project in Unity.
Go the Assets menu in the top left of the Unity Window. In the drop down menu select
Import Package > Custom Package Browse to where you put the SDK file and select that file and press open.
After a short time a window will open, select Import in the bottom right of that window and the SDK will be installed into your project.