help with facebook thumbnails

Hey guys,
im new on this fantastic platform and i wish i had discovered it earlier. In any case although i am very happy to run my blog im getting very frustrated because of some bugs i cannot fix! In particular, the facebook thumbnails that are very damaging my blog because it does not appear the image related to the posts but a strange images that remember me something of wordpress.com! furthermore many pages/categories do not work ant it says error 404 page not found. I tried several plugins without success and unfortunately i am not so confident with css. i don`t know what to do, please help me guys!