There is a jarsigning tutorial on this site, or please search the forums as there are tons of answered posts about this. In fact, if I remem correctly there is (or was) a code-signing forum! It's always a good idea to search before posting as it saves us having to say the same things ad nauseum.
The tool you will be using is packaged with the JDK and is called (ironically) 'jarsigner'.