I wanted to ask if a girl could get pregnant before her first period when the sperms do not enter her vagina. I pulled out before ejaculating.
What you are saying is that you've committed fornication by having sex with a girl. "Marriage is honorable among all, and the bed undefiled; but fornicators and adulterers God will judge" (Hebrews 13:4). You have no business putting your penis into a girl whom you are not married to. Worse, you did so with a girl who is young enough not to be physically mature.
I don't know your ages, but in most places a man who is significantly older than a girl who is a minor can be prosecuted for statutory rape. What that means is that when an older man has sex with a minor girl, it doesn't matter whether the girl consented or not. It is considered rape under the law because the girl is deemed too young to understand the consequences of what was being done.
Your concern about whether she could get pregnant tells me that you are more concerned about hiding the evidence of your sin or avoiding responsibility for your sin than the fact that you took advantage of a young girl for your sexual pleasure and sinned against her and God.
So let's talk about the facts. A girl has her period two weeks after she releases an egg, if that egg does not become fertilized. Since a period always follows the release of an egg, you cannot know when a girl will release her first egg until after the fact.
Even though you did not ejaculate while your penis was inside her, it does not mean you did not leave sperm behind in her vagina. When you are sexually aroused, you drip pre-ejaculate fluid (also called pre-cum). It is possible for sperm to be present in that fluid. Also, even if you ejaculated outside her vagina, it is possible for some of the semen to come in contact with her vagina. Sperm swim and they don't care how they get to vagina.
The odds are very tiny that you might have gotten her pregnant, but when sex is involved, it is never zero. The few sperm that might be present might also not survive the journey. However, it can happen. I knew a young man many years ago who thought he could withdraw before ejaculating, and he did, but she got pregnant anyway even though they did it only once.
Regardless of whether you got her pregnant or not, the real point is that you did something that was wrong. You need to change.