To sign your MIDlet's JAR you need to obtain a code-signing certificate. Note, however, that SSL certificates are not acceptable to sign code. As a developer, you have to be aware what certificates will be acceptable for your target device. In this example, the JAR will be signed by using a certificate which has been signed by Verisign and purchased through the normal channels.
Verisign offers several types of code signing certificates, each for a different application. For MIDlet suites signing the Sun Java Signing Digital ID type is required. Remember that Certificate Authorities (CAs) will usually charge a fee for issuing the certificate and they have a limited validity period. Verisign will have to verify your identity usually by requesting some proof of your identity and organization.
When signing the JAR file, the certificate will include an identifier of you as the developer or your company, so that the MIDlet can be associated with the signer. This is also an important input for technical non-repudiation.
For experimentation purposes, it is possible to create your own "self-signed" certificate and load it to the emulator. However, this approach will not work in actual S60 and Series 40 devices since the set of root certificates is closed.
For a description of the signing procedure, see section Signing procedure using command line utilities.