Won't let me start up game after I put in a mod/put in mod loader

First time ever modding this thing and I don't really know how to do it but when I first put in mod loader it didn't work but I suppose it is the second time but it didn't. And whenever I put a mod into "normal" or "priority" it just says "does not have asset folder" but it never came with one. I need some help with this.