(1) Basic
\begin{align}
y = f(x) \\ %automatically numbering, "\\" means "change the line"
y = g(x) \nonumber \\ % "\nonumber" means "do not numbering this equation"
y = \frac{1}{x} \nonumber
\end{align}
(2) Alignment
\begin{align}
y &= f(x) \nonumber \\
&= T_{0}+ (T_{0}-T_{h})e^{-x} % "&"s are automatically aligned them among themselves.
\end{align}