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The opening of our vagina is protected by a thin piece of membrane called HYMEN. Its purpose is to protect female babies from vaginal infections and as we get older, it loses its aim.
When you get your first period(it's called menarche) , the strenght of the blood flow will open the tissue.
(Sometimes the hymen doesn't open and this leads to health issues bcs period blood and vaginal secretions can't get out.)
SO because of the remaining amount of this tissue we bleed the first time we have coitus. But the bleeding should be really minimal, like a paper cut.
As for the pain in your first intercourse (penis in vagina) this is supposed to be really low. You need your partner to stimulate your clytoris to feel pleasure, we don't have orgasm by stimulating the vagina.
Hope this summary helps some of the girls in the comment sections <3