My equation is very long. How do I get it to continue on the next line rather than go off the page?
There are a couple ways you can deal with this. First, and perhaps best, is to rework your equation so that it is not so long; it is likely unreadable if it is that long.
If it must be so, check out the AMS Short Math Guide for some ways to handle it. (on the second page)
Personally, I'd use an align environment, so that the breaking and alignment can be precisely controlled. e.g.
\begin{align*}
x&+y+\dots+\dots+x_100000000\\
&+x_100000001+\dots+\dots
\end{align*}
which would line up the first plus signs of each line... but obviously, you can set the alignments wherever you like.