What you do is set your character on the front page. You put your name and realm and all that. It will take a couple of seconds to load, once it does you use the left navigation bar to see the achievements in there. It will show you which you are done with and which you aren't done yet as well as your progression bars. There are options in those nav bars to only show which you haven't completed yet too. When you click on an achievement on that nav if we have a guide for it it will take you to it, if we don't it will take you to a submit page.

Also, once you set your character on the top nav you can also click on the Suggested tab and it will show you a list of achievements you are really close to completing. You can reset the character at any time logged in or out at the front page. You can put any character if you want to check on alts or friends =)