When did the first stars form?
The very first stars were born when the Universe was a few hundred million years old. They were very different from today’s stars; they contained no elements besides hydrogen and helium.
The first stars formed from contracting gas clouds of hydrogen and helium. Their composition gave these stars special properties: they were extremely massive, and lived only brief lives, converting part of their nuclear fuel into heavier elements. When they exploded as supernovae, interstellar space was enriched with these heavier elements. First-generation stars (also known as Population III stars) have not yet been observationally confirmed, but future instruments like the James Webb Space Telescope and the Extremely Large Telescope (ELT) may succeed in detecting them.
In the early Universe there must have been one star that was the very first to start shining! For a brief while, it was the only star in the entire Universe. That happened over 13 billion years ago.