Sometimes I'm aroused after seeing someone's penis, but I believe I am straight

I'm a girl and I've never masterbated but I really want to but I just don't have the guts to stick my finger up there it's a bit unnerving someone help

Hi
It is normal to get excited by doing anything that turns you. Many guys experiment with each other and obviously enjoy doing it. Discovering your sexuality is part of growing and developing and for many guys and girls experimenting in same sex activities is very common.
Soon enough you will know if what your doing is just a bit of curiosity or if the feelings you have are more than experimenting. There is nothing wrong with you doing this and it will help you to figure out your preference.