Your first port of call should be the official manual which you can download from the Rhino website. Follow through that and it will give you a basic understanding. Two sets of videos I would recommend are;
PolyPlane | Archive
The archive of previous free video tutorials at PolyPlane.com
Est. reading time: 4 minutes
When you get more comfortable with it there are tons of more specific videos on YouTube, including these by Simply Rhino in particular;
Simply Rhino 3D Tutorials & Events
Welcome to the Simply Rhino channel - we look to curate & produce unique video content to help you learn and improve with Rhino 3D software and learn about applications for Rhino and Grasshopper. If you enjoy what you see in our videos and want...