Sidekickery is a vital job in the Renaissance. Your first order of business is to tell your hero why you're so awesome. You should do this with great panache - song and dance is a must. After you've gotten the gig, your next task is to tell your hero why he's so awesome. (Heroes tend to have identity crises. They're high maintenance like that.) After that, finding some other talent or way to help out is a good idea. If you can fly, you're pretty much set. Otherwise, you should try to work on bossing other sidekicks around. That way you establish some authority. It also helps if you have a great sense of humor, know how to train your hero for battle, or hold a position as the king's right hand man.
If all else fails, being chubby and cute works just fine.