The position of "Name of Y" can be changed using a \multicolumn. For the alignment, you can use the S column type provided by siunitx (the package offers many additional possibilities). To have columns of equal width you could use the tabularx package; to make it cooperate with siunitx, some additional work is required by adding
\begingroup
\catcode`\_=11
\catcode`\:=11
\gdef\tabularxcolumn#1{%
>{\__siunitx_table_collect_begin:Nn S{} }%
p{#1}
<{\__siunitx_table_print:}%
}
\endgroup
which is nothing more than an update (using the new syntax of siunitx) to Martin Scharrer's answer to How to use siunitx and tabularx together?. In your case, however, the table seems to be really wide, so you could additionally rotate it using sidewaystable from the rotating package (You force the reader to change the reading direction, but the table values are much more clearer).
A complete example:
\documentclass{article}
\usepackage{booktabs} % this package promotes good tabular style
\usepackage{caption} % for customising caption style
\usepackage{siunitx} % for decimal alignment
\usepackage{tabularx} % for columns of equal width
\usepackage{rotating} % for rotating objects
% make tabularx and siunitx cooperate
\begingroup
\catcode`\_=11
\catcode`\:=11
\gdef\tabularxcolumn#1{%
>{\__siunitx_table_collect_begin:Nn S{} }%
p{#1}
<{\__siunitx_table_print:}%
}
\endgroup
\begin{document}
Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor
incididunt ut labore.
\begin{sidewaystable}
\centering
\captionsetup{singlelinecheck = on, justification = centering, labelfont = bf}
\caption{Aaaaaaaaaaaaaaaaa}
\begin{tabularx}{\linewidth}{c*{10}{X}}
\toprule
& \multicolumn{10}{c}{Name of Y} \\
\cmidrule(lr){2-11}
{Name of X} & {1} & {2} & {3} & {4} & {5} & {6} & {7} & {8} & {9} & {10} \\
\midrule
2000 & 1.23 & 45.617 & 14.15 & 11.11 & 11.111 & 555.55 & 555.55 & 66.666 & 777.7 & 8888.8 \\
2000 & 12.3 & 456.17 & 1.415 & 11 & 111.11 & 55.555 & 5555.5 & 66.666 & 7777.7 & 888.88 \\
\bottomrule
\end{tabularx}
\end{sidewaystable}
Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor
incididunt ut labore.
\end{document}
Some other options would be to use \resizebox that allows you to scale the table:
\resizebox{<width>}{<height>}{<content>}
the ! modifier leaves the natural dimension unaltered. (If the table is too wide, perhaps a portrait display would be better than scaling it):
\documentclass{article}
\usepackage{booktabs} % this package promotes good tabular style
\usepackage{caption} % for customising caption style
\usepackage{siunitx} % for decimal alignment
\begin{document}
Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor
incididunt ut labore.
\begin{table}[!ht]
\footnotesize
\centering
\captionsetup{singlelinecheck = on, justification = centering, labelfont = bf}
\caption{Aaaaaaaaaaaaaaaaa}
\resizebox{\textwidth}{!}{\begin{tabular}{c*{10}{S}}
\toprule
& \multicolumn{10}{c}{Name of Y} \\
\cmidrule(lr){2-11}
{Name of X} & {1} & {2} & {3} & {4} & {5} & {6} & {7} & {8} & {9} & {10} \\
\midrule
2000 & 1.23 & 45.617 & 14.15 & 11.11 & 11.111 & 555.55 & 555.55 & 66.666 & 777.7 & 88.888 \\
2000 & 12.3 & 456.17 & 1.415 & 11 & 111.11 & 55.555 & 5555.5 & 66.666 & 7777.7 & 888.88 \\
\bottomrule
\end{tabular}}
\end{table}
Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor
incididunt ut labore.
\end{document}
There's also \scalebox{<x-factor>}[<y-factor>]{contents}, but using this you need to by trial-error determine the appropriate factor(s).
Another option reducing \tabcolsep and still using \resizebox:
\documentclass{article}
\usepackage{booktabs} % this package promotes good tabular style
\usepackage{caption} % for customising caption style
\usepackage{siunitx} % for decimal alignment
\begin{document}
Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor
incididunt ut labore.
\begin{table}[!ht]
\setlength{\tabcolsep}{3pt} %length between columns; default = 6
\footnotesize
\centering
\captionsetup{singlelinecheck = on, justification = centering, labelfont = bf}
\caption{Aaaaaaaaaaaaaaaaa}
\resizebox{\textwidth}{!}{\begin{tabular}{c*{10}{S}}
\toprule
& \multicolumn{10}{c}{Name of Y} \\
\cmidrule(lr){2-11}
{Name of X} & {1} & {2} & {3} & {4} & {5} & {6} & {7} & {8} & {9} & {10} \\
\midrule
2000 & 1.23 & 45.617 & 14.15 & 11.11 & 11.111 & 555.55 & 555.55 & 66.666 & 777.7 & 88.888 \\
2000 & 12.3 & 456.17 & 1.415 & 11 & 111.11 & 55.555 & 5555.5 & 66.666 & 7777.7 & 888.88 \\
\bottomrule
\end{tabular}}
\end{table}
Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor
incididunt ut labore.
\end{document}
