How to create a multi-row equation?

\documentclass{article}
\usepackage{amsmath}

\begin{document}
\begin{equation}\label{M-ary PSK}
S(t)= \sqrt{\frac{2E}{T}}\cos(2\pi f_{c} t + \frac{2\pi}{M}i),\qquad
    \begin{aligned}
&i=0, 1, \dots, M-1\\
&0 \leq t \leq T
    \end{aligned}
\end{equation}
\end{document}

changes (in comparison t your code snipped):

• \cos instead cos (cos meaning three variable: c, o and s)
• conditions follows main equation in aligned environment from package amsmath
• instead of ... is used \dots

edit: or as pointed Ari Brodsky in comment below:

\documentclass{article}
\usepackage{amsmath} 

\begin{document}
\begin{equation}\label{M-ary PSK}
S(t)= \sqrt{\frac{2E}{T}}\cos(2\pi f_{c} t + \frac{2\pi}{M}i),\qquad
    \begin{gathered}  % <-- changed
i=0, 1, \dots, M-1\\
0 \leq t \leq T
    \end{gathered}
\end{equation}
\end{document}

You can just set the dual conditions using an array:

\documentclass{article}

\usepackage{amsmath}

\begin{document}

\begin{equation}
  S(t) = \sqrt{ \tfrac{2E}{T} } \cos \bigl( 2\pi f_c t + \tfrac{2\pi}{M} i \bigr), \quad 
  \begin{array}{c}
    i = 0, 1, \dots, M-1 \\[\jot]
    0 \leq t \leq T
  \end{array}
\end{equation}

\end{document}

Consider using \dots rather than ....