The film came out in 1992, and the series came out in 1997. The story of the film takes place before the series, so the series would be the "sequel." Some changes were made when the series came out, such as:
Buffy is a high school sophomore instead of a senior;
vampires no longer give Slayers cramps when in close proximity;
Slayers no longer have a distinctive birthmark;
Watchers do not live hundreds of years nor train Slayers for many lives.
Since the Buffyverse Wiki has the series as the ultimate canon source, the film is considered non-canon because it contradicts the series in a lot of ways. The overall story in the film, however, is mostly considered canon, because the series mentions Buffy burning down the school gym in Los Angeles. You can see the canonical version of the film in the comic miniseries The Origin.
