They typically—at least in my experience—like to test people out for three weeks. Those first three weeks are when they decide whether or not to keep you, and so is the best time to show one thing in particular, and that is [i]effort[/i].

If you're a little unskilled at first, that's normal. You just have to show that you'll do your best to get past that point of being unskilled.

I've trained a lot of dishwashers since I started at my workplace, and I can tell you, almost all the people we didn't keep after the three weeks were the people who were just going through the motions, not really [i]trying[/i], just doing. Gotta be constantly at it, keep working till the jobs done, and then see if there's another job you can do. Some workplaces value speed, others quality, but you should expect to not be the best on your first day.

Like mdk said, they look for potential, and you need to show that by demonstrating that you are focused and looking for improvement.

Have fun!