Where do you get the mitochondria from?

The mitochondria in your body comes from your mother, but in some rare cases, it can come from someone's father. Endosymbiosis theory is a theory of how mitochondria (and other cell organelles such as choloroplasts) came to become part of the eukaryotic cell. Since mitochondria have their own DNA, it is theorized that they were originally independently living organisms that have been engulfed by a eukaryotic cell, much like choloroplasts were originally photosynthetic bacteria swallowed by the ancestor of plants. Once they had been swallowed, instead of being digested, they became part of the eukaryotic cells instead. The endosymbiotic event happened billions of years ago. Currently there is no free living mitochondria in the environment, so you did not capture them yourself, but instead inherited the mitochondria from your mother.

You do have mitochondria when you're born.

They're already inside your mother's egg cells before fertilisation. As each egg cell divides to form the embryo, each new cell gets a share of the mitochondria. New mitochondria are formed by division, in a process similar to cell division.

As the embryo develops, each new cell gets a share of the dividing mitochondria so that when you're born, every cell already has mitochondria.

Your mother got her mitochondria from her mother, and so one through the female line. Male spam cells contain mitochondria inherited from the male's mother, but these are lost when an egg is fertilised, so only the egg's mitochondria remain.

You clearly don't have the first clue what the endosymbiont theory actually says. The symbiosis isn't established from scratch every generation. It happened ONCE, many many many years ago. Your mitochondria are descended from those that were in your mother's egg cell, which was fertilized to become you.

Next time, try listening to ALL of the worlds your biology teacher says.