I'm actually 16 years old but I tell people I'm 13 when I talk to them online.
That can be really serious. Why do you lie about your age online?
I know I can get more girls if they think I'm younger.
While it's good to socialize online it's wrong when you lie to others when you do so. Additionally, as a teenager, it's especially important when you are trying to find a romantic partner for them to be the same age as you are or you'll risk getting in legal trouble. You should always be honest with your age online to prevent any misunderstandings arising from a large age gap.
I think it's stupid since some girls that age in my school flirt with me and I like them back.
It would better to try and get girls that are around your own age. Disguising yourself online can be harmful and put you and them in uncomfortable situations. It's healthier to be in relationships with people on the same maturity level as you.