\begin{center}
\begin{minipage}{.7\linewidth}
jajasdjsdjsd sdj asj
\end{minipage}
\end{center}
or you could use
\begin{quotation}
jajasdjsdjsd sdj asj
\end{quotation}
(which allows page breaking)
or you could use
\begin{abstract}
jajasdjsdjsd sdj asj
\end{abstract}
which is more or less the same, but writes abstract at the start.
In short: Plain TeX and LaTeX are different macro packages that define different formats. So, in general, it is not possible to use LaTeX packages on top of Plain TeX. For a short introduction to all this rather specialized jargon you may check this blog post
It is possible to use some LaTeX packages on Plain TeX (graphics, color and hyperref, for instance), by faking some functionality of LaTeX; only that necessary to load and make the packages operative. Yet another macro package built on top of plain, eplain, has a built-in ability to load some basic LaTeX macro packages.
\beginpackages
\usepackage{graphicx,color}
\usepackage{url}
\endpackages
It is also possible to use some Plain TeX functionality in LaTeX documents with the plain package and the plain environment provided. Still, it's not the whole thing, but works for many things.
\usepackage{plain}
\begin{plain}
[plain stuff]
\end{plain}
You can find all this stuff in CTAN, which contains lots of thing TeX related. Beware though that you need to learn how to navigate in the tex-archive tree.
The MikTeX package manager deals mostly with LaTeX packages, so you may need to learn how to install Plain macro files manually.
Best Answer
You can now choose between
and
An empty line in the text will end the line.