There are certain restrictions for accessing certain APIs from developer’s MIDlets. In these cases it is possible that the user will either be prompted for confirmation or the access is blocked, resulting in a SecurityException. By Signing a MIDlet these security prompts can be reduced. Other way to achieve this is to manually change the API access settings on the SDK.
Signing by the operator or manufacturer (root/node) certificate will remove the prompts completely, but may require close collaboration with them.
For more information, refer to Java ME permissions.
What certificates to use for Signing : you can get certificate from certificate authority or certification authority (CA) or else follow the Java Verified.