Your first living cell came from the fusion of two gametes from mom and dad, otherwise known as a sperm and an egg cell. Those two cells each contain one half of the DNA required to make a functioning cell, so when they fuse together to make a single cell, that cell has a full set of DNA it can read to make more copies of itself.

If your asking where the first living cell ever came from? No one has a capital T True answer because we haven't been able to recreate the event in a lab, instead we have some solid hypothesis based on stuff we have seen. The first hypothesis believes kind of three main things: #1 is that the first cell probably didn't look anything like our cells, because it would have been super simple. #2 it probably didn't actually use DNA, and instead might ve used a similar molecule called RNA, which naturally likes to fold in on itself, protecting it from being damaged by the environment, but still acting like DNA as a blueprint for cell processes. #3 is that the cell would have come about randomly in the sea of primordial soup after many many many many many years of random chemical reactions. Eventually the right sequence was hit, starting a reaction that didn't just reach the end and stop, but instead caused more reactions to occur and store energy to restart the process again.

That's SUPER high level, barely scratching the surface, barely covering any of the complex things that would have needed to happen to keep those chains going for #3, but hopefully it helps answer your question.

The other hypothesis is something intelligent created the first cells. Where'd that intelligence come from? Ask your leader of choice (priest, rabbi, imam, etc.)