I'll asume you mean via Xbox Live....?
1.) Hosting a game simply means you started the game.
To host a game (I'll assume you mean custom game) select custom games on the main menu. Once in custom games send your friends or recent players invites using your jewel in the center of your controller. Once everyone arrives you can choose map, gametype, player traits, etc...
2.) To see if you are hosting a game look into the party details to see who is Party Leader.... or if you are still in a lobby look to the top of your screen and it will say Party Leader - _______?
Hope I was of some help .... if not e-mail me and ill walk you through it using me gamertag on Xbox Live