The sidewaystable environment is a floating (and rotated) environment, but it does not in itself provide instructions on how to typeset tabular material. For that, you need to specify a tabular, tabularx, tabular*, etc environment inside the sidewaystable environment. (In other words, the sidewaystable environment behaves exactly like the "ordinary" table environment does, except that it rotates its contents and places them on a separate page.)
For instance, to suppress the horizontal whitespace at the left and right-hand ends of a tabular environment that has four columns (the first left-justified, the remaining three centered), you should specify something like
\begin{tabular}{ @{} lccc @{} }
...
\end{tabular}
The @{} group tells LaTeX to suppress the horizontal whitespace.
If you're using a tabular* environment with a width equal to \textwidth, i.e., the full width of the text block, you could also specify
\begin{tabular*}{\textwidth}{ @{\extracolsep{\fill}} lccc }
...
\end{tabular*}