With an up to date LaTeX installation,the UTF-8 encoding and the "textcomp" package are used by default. This makes the input of the MICRO SIGN simple:
\documentclass{article}
\begin{document}
The length is 10\,µm.
The length is 10\,\textmu m.
\end{document}
Mind, that the µ in the example is the Unicode character 00B5 MICRO SIGN.
For the similar looking character 'μ' (03BC GREEK SMALL LETTER MU) as well as for all other Greek letters, Unicode input is supported "everywhere" by loading the package textalpha (see its documentation for limitations) or in text parts marked as greek by loading Babel with, e.g., \usepackage[greek,english]{babel}.