Just diagnosed with Hypothyroid, worried not sure what to do?

I just had my annual physical and my doctor called me that my thyroid TSH level was very high meaning it is underactive I guess. He prescribed Synthroid to start. Is this a serious disorder? I don't know much about it. Is there dietary changes I can do to help? I am just looking for info from people who have this or good websites for research. I appreciate it. I am worried about this.