Validation errors

I am in the process of trying to make my sites better, and I found many validation errors in them, which I have no idea of how to fix… however, most of them have to do with the links from affiliates and the code in the plugins. Are these errors something that will affect me in the long run? Should I get the plugin authors to help me? (if I can reach any of them), or should I just let the errors be? If not, is there a place you can recommend to help me fix the errors myself, since I can’t afford to pay anybody right now?

http://www.yourpetsuniverse.com
http://www.thefishtankplace.com
http://www.lifewithfengshui.com