How do you take a girl's virginity?

The simple answer is by having sexual intercourse.

The Word "virgin" means a woman who has never had vaginal intercourse (though the Word is also applied to males as well). When a penis is inserted into your vagina, then you "lose" your virginity. Yes, inserting "just the tip" of the penis is still having sex, and yes, you could very easily become pregnant from doing that, because often the first few drops of semen contain the most sperm.

For men, it is the same; by having sexual intercourse.

Hymen

Some people confuse "losing virginity" with losing a bit of tissue that some (not all) women have over the entrance of the vagina. This is called a hymen, and is just a piece of membrane. Not all women have a hymen to begin with, and any sort of rough activity can tear this tissue - riding a bike, falling onto the top rail of a boy's bike, horseback riding, sports, being kicked between the legs, masturbation, medical exam or procedure, etc.