I can't seem to comment on someone elses answer, only on my own answers. Am I missing something or do I not have a good enough reputation yet to comment on others' answers?
By participating in the site, you earn reputation points. Once you've reached a high enough reputation level, you're given additional abilities within the site. See the FAQs for a full list of reputation levels and appropriate user features.
Commenting on another person's answer, for example, requires 50 reputation points. The more comments and answers you leave (that get accepted and/or up-voted by the community), the higher your reputation will get.