Every Yom Tov, just like Shabbat, begins sometime between early evening (“Plag HaMincha”) and just before sunset – for more details on this, see here.
When it comes to Shavuot, the same holds true – candles should be lit and all melacha (forbidden activity) should be stopped before sunset (8:49pm this year). However, there is a well-known custom, dating back to the 16th century at least, to wait until nightfall (10pm this year) before making Kiddush. Others have the custom to wait until nightfall before davening Maariv (prior to Kiddush).
While this is a praiseworthy and prevalent custom, one who feels unable to wait until nightfall may bring in Yom Tov earlier (from 7:23 pm if necessary), daven Maariv, make Kiddush and eat.
As always, feel free to be in touch if you have any questions – firstname.lastname@example.org.