system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What actually decides when I'm born if I'm a boy or a girl?

Sperm, egg or other?
gpt
Your dad's sperm.

Your DNA (the blueprints to build a complete human), is broken up into like little chapers, called chromosomes. Everyone has two copies of all 23 chapters. Or you have 23 chromosomes from mom (the egg) and 23 from dad (the sperm) for a total of 46 (you need all 46 for everything to work but thats another ELI5).

Now of those 23 (remember you have a copy of each) only one determines sex, #23, the sex chromosome. 

All fetuses are defaultly female, the only thing that makes them into a male is two certain hormones at very specific times. The codes to make these hormones are found on a chromosome called Y. 

So lets do some deductions. If Y is the chapter than makes the "turn this baby chick into a baby dude" signal; and your mom isn't a dude, then she doesn't have Y. She has the Y's female copy, called X. Actually remember she has two X's (every chromosome has a copy). She got one X from her mom and one from her dad. No Y, so no signal to make a dude, so when she was an fetus it defaulted to a female, the natural sex.

So when her eggs are formed and she gets to give one of her copes, she can only give an X, thats all she has. But you dad, he's a dude, so he as an Y, and since he's from a man and woman, the other #23 he has is an X. So when his sperm form there is a 50/50 chance it gets an X copy or a Y copy.

So its the sperms, with its X or Y than when it fertilizes the egg, which can only have an X, that makes the sexual determination.