I have an opportunity to shoot pictures at a wedding, and I was wondering if anyone has any experience in this and has any tips. I've never done this before (I've done portrait photography before, just not a lot), and I want to come off as professional...

It all depends on the equipment you've got.If all you have is a point & shoot then don't try. You'll be dealing with people who are emotional involved and any tiny imperfection will them down on you.Equipment: SLR camera (preferably a Digital, if not make sure you have a wide array of film 200-1600 ISO, a Medium format camera is also a plus.), Wide angle lens (16-35 mm), Medium lens (35-70 mm), Long lens (70-200 or even 300 mm), On camera bounce flash. If you can set up studio lighting and back drop you will benefit greatly. If you have two cameras the transition between lenses and ISOs will be easier on you since you can set either one up differently.Tipson't be afraid to get close. Robert Capa said that if your photos are no good then you're not close enough. Use your flash as little as possible, natural light is always the best. If you shoot with a digital SLR then adjust your setting according to the lighting situation you will be in.