I am recently new to this website and I would like to know how to link my Live account with my achievements that I gathered for Halo 3 and other Halo games so that it apprears on my account.
Thanks for any support in advance. 
-FMF
You don’t need to link it. It should automatically do it for you. Since you’re new, maybe you could wait a few days, and see if it’s there. Welcome to the site!
IF you want to link your stats to your account here one waypoint, when signing up use your windows live id associated with your account and the password for the account and it should work to my knowledge.