The year zero does not exist in the Anno Domini (AD) system commonly used to number years in the Gregorian calendar [1], so the first decade was years 1AD-10AD (ten years). This means current decade started on January 1st 2001. and ends with December 31st 2020.
From Wikipedia Decade[2] article:
There are two main methods of counting decades in recognition. One, called ordinal, counts decades starting with the first year 1 CE (For example, the years 1981–1990 is referred to as the 199th decade or the 9th decade of the 20th century), while the other, called cardinal, groups years based on having the same digits (For example, the years 1980–1989 is referred to as the 1980s, or commonly known as the eighties).
From Wikipedia Decade[2] article:
There are two main methods of counting decades in recognition. One, called ordinal, counts decades starting with the first year 1 CE (For example, the years 1981–1990 is referred to as the 199th decade or the 9th decade of the 20th century), while the other, called cardinal, groups years based on having the same digits (For example, the years 1980–1989 is referred to as the 1980s, or commonly known as the eighties).
1: https://en.wikipedia.org/wiki/Year_zero
2: https://en.wikipedia.org/wiki/Decade