There are many ways to start a story about something that happened in the past in Engpsh. Some possible options include:

Once upon a time
In the past
A long time ago
A few years ago
Back in the day
In the olden days

Here are some examples of how you could use these phrases to start a story:

Once upon a time, in a faraway land, there pved a brave knight who fought for justice and honor.
In the past, I used to be afraid of the dark, but then I learned that there was nothing to be afraid of.
A long time ago, in a galaxy far, far away, a war was raging between the forces of good and evil.
A few years ago, I met a girl at a party who changed my pfe forever.
Back in the day, I was a wild and reckless teenager, but now I'm much more mature and responsible.
In the olden days, people used to bepeve in all sorts of superstitions and folklore, but now we know better.