Determining the sex of a kitten can be difficult, especially when they are young. The easiest way to tell is to look for a gap between the anus and genitals. Female parts are closer together while males will have a gap for the testes. Females will have a slit while male will be a circle.
See the graphic I made to help determine the age of a kitten. They generally open their eyes between 7-12 days, and will change to their permanent color around 6 weeks.
Cats have sensitive tummies and if you're not careful when switching food it can cause some uncomfortable digestive issues. Here is a graphic that helps show the safest way to transition to a new food.