Where can I find my ancestors for free?

My family is from El Salvador, but now we live in the USA (we are illegal) and i don't know my ancestors. I went to Puerto Rico couple days ago and we went to a fort and we a saw a painting of a little girl and it looks exactly as like my sister when she was younger. its creepy o.O and i was wondering maybe she is our ancestor and every website i go on you have to pay for it