what age is the normal age to be interested in sex for girls

From what you say you are unsure of your sexuality. That is not uncommon for people your age and a little older. Even though your friends may be at it like rabbits there is nothing wrong with lack of interest in sex. Essentially libido (interest in sex) is a hormonal thing. You may well not be fully past puberty at your age. Whilst in boys puberty triggers huge sexual desire the same cannot always be said of girls.
You say that you feel scared at the idea and insecure about your appearance. For girls there is a certain amount of risk with having sex, there is the possibility of pregnancy or STIs, so you need to understand how to manage and reduce those risks. Forming relationships is also very stressful and can leave emotional scars for many years.
The good news is that there are ways you can improve your appearance and begin to feel more confident. If you have an older female friend or relative they will give you advice.
Don't be worried about the situation for now, there is no harm in waiting a few years to see how you feel then. There is no law saying that you MUST start having sex or forming relationships at 16!