Congratulations, I think dogs do better if you keep them on the same food at least a week, and even better a month. That way you can tell if any upsets are from the food or something else. People have strong opinions here on food; some people love Science Diet, while others hate it. I try to look for a food that has a named meat as the first ingredient. Here's a site that will help teach you some basic in nutrition.
The Dog Food Project - Identifying better products I don't think it's good to switch too much the first few months, and if you do switch, add the new food to the old, and do it very gradually.