Let's begin at once,it's already late.

It's already late
这里的it功能和it's raining/it's Monday等句型一样,这些都是'无主语句",it除了是代词'它'的含义,还一个功能就是补充担当无主语句的'主语'.只有语法功能,不代表其它的含义.（有点象占位符)
Let's begin at once
at once=right away.
这是祈使句,也是无主语句.
字典:
used in the normal subject position in statements about time, distance, or weather. (上面的例子)