it means "if I want"
It means "if I want". Si is the Spanish word for if. Quiero is the yo form of querer, which means "to want" and yo means "I", so quiero is "I want". If it was something like sí quiero (notice the accent on the i in sí), it would be something like "yes, I want" since sí is the Spanish word for yes.
¿Y si te quiero?
"Si te quiero" in English translates to "I love you."
"Los quiero" in Spanish means "I love them" when referring to a group of people.
"si quiero ser tu novia"
"I want to..."
quiero = I want
I want or I love
te quiero
"No quiero hacer" in Spanish translates to "I do not want to do."
Quiero ir means "I want to go".
"Lo quiero" in Spanish translates to "I want it" in English.