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
https://www.youtube.com/playlist?list=PLAT6cUbaGgg9L2XGMn2fr7JrZsucxieON
When you get more comfortable with it there are tons of more specific videos on YouTube, including these by Simply Rhino in particular;
https://www.youtube.com/channel/UCRBjWaXSeqTP89FWVv26QxA/videos