To apply for 6 nations tickets through the official outlet you need to go through the National Rugby website of the nation hosting the game.
E.g England v Wales you need to go to www.rfu.co.uk
Bear in mind some of the tickets for the games have already gone on sale and are already sold out and now being relisted on other ticket sites for big bucks
For more info on how to get tickets you may want to start here....
Otherwise try googling the match you want tickets for and then you should get the results of all the ticket auction/resale sites.
If you are a serious rugby fan, it also helps joining the fan club as you get access to advanced ticket sales/priority ballots though you will pay an annual membership fee for this.