Different units and branches require different things. In the Infantry you’ll want to show up physically fit with a Ranger tab, able to execute the things you learned in the school house, and as others have mentioned: a positive attitude. What that means to me is that you are eager to lead, ready to learn from others, and can execute all of the specified and implied tasks given to you.  Things that you can do before you show up: contact the unit S1 and find out what information your future BN CDR wants in your letter of introduction (sent via email). Besides that, perhaps ask the S1 for the quarterly training guidance so that you know what you will be doing when you show up.