Where is the Keytool application?
keytool is a tool to manage (public/private) security keys and certificates and store them in a Java KeyStore file (stored_file_name.jks).
It is provided with any standard JDK/JRE distributions.
You can find it under the following folder %JAVA_HOME%\bin.
keytool is part of the standard java distribution.
In a windows 64-bit machine, you would normally find the jdk at
C:\Program Files\Java\jdk1.8.0_121\bin
It is used for managing keys and certificates you can sign things with, in your case, probably a jar file.
If you provide more details of what you need to do, we could probably give you a more specific answer.
For me it turned out to be in c/Program Files/Java/jdk1.7.0_25/bin (Windows 8). A more general answer to this question is that it will most likely be in the bin sub directory of wherever your jdk is installed.