You probably already know this, but like Lawrence said make sure you bring your PPE (do NOT forget safety glasses), welding jacket, hood and gloves. I know this sounds obvious, but when I worked the shop floors, I watched a number of guys come in to test who didn't bring a hood or gloves. Needless to say those guys didn't make it. You sound like you are already somewhat familiar with this company. Maybe you may have a good idea of what welding processes they use and material types that they weld on. If so, familiarize your self with them, and maybe most important. Don't go in with a I know this and I know that attitude. Show them that you have abilitiy to be a good worker and that you can be flexible and are willing to learn.