Sign a Java Applet so that warnings are not generated in IE and FF

Our website is using the ZUpload Open Source Java Applet for uploading files. It is currently signed but it generates a warning when used in IE and FF. The task is to remove this warning. We do not know how to achieve this and will require guidance all the way. We understand that a certificate must be purchased. We will require advice on purchasing this certificate. We will supply the Java Applet with source code, and require that it is returned to us in a condition where it no longer generates warnings when used in IE6, IE7, and FF. We can purchase the certificate but will require advice on which certificate to purchase and where from. We require a developer who will take full responsibility for the task and guide us all the way. *** Please do not post standard replies telling us how great you are and nothing else. We want to know if you have done such a task before and that it is within your skills, and if you can guide us all the way and take responsibility. ***