Guide to making a good first impression:
http://forum.becomealivinggod.com/t/i-m-laying-my-cards-down-on-the-table-so-to-speak/2551/3
At your age, tone it down on the smiling, act very serious, people like that - they associate happy youngsters with irresponsible attitudes. Also, don’t try to impress people with high-status clothes or accessories, again they’ll think it’s tacky or even that you stole them, or are irresponsible in some other way.
And NEVER show someone you’re cleverer than them, most people in most mundane lines of work are looking for a younger person who’s like them but just that bit less competant, which is why management of big organisations always decays, no-one wants to be surpassed by the kid they hired.
Guide to coming across as cool:
Hope some of this helps! 