Getting make-up right is not easy. You don't want it to look like something a man in drag might wear; you want it to look feminine and age-appropriate.
Think of how young girls look when they are first trying out make-up and how over the top it can look. You need to make sure you practise before you go out in public so that you don't have this prepubescent look for your everyday wear.
Make-up should be applied carefully and not be too thick. If you don't feel confident about putting it on, the best thing to do is to apply only a little - just start with some lipstick. It helps to give some colour to the face and will help you feel more feminine.
Going for more neutral shades will not give too heavy a look. Many beauty counter assistants are usually quite happy to demonstrate their products on you whilst giving you some lessons in what products work well for you.
TIP:
Less is more if you are not sure what you are doing. Practise at home before you go out.