New to WireShark & Networks

I run a small business and have network problems from time to time. I am very interested in broadening my knowledge and also get to grips with WireShark.

As this is a new field and I have limited knowledge about Networks, please can someone direct me in where to start, maybe literature, videos and/or websites that will help me. I've read through the WireShark Manual and looked at some of the linked videos, but I think I need to get a grip on Networks first.