First of all you have to put things in perspective.
Every single person has some insecurity about something in their lives. It's not a unique problem to you.
Secondly you have to address it with the following question.
Can I change (whatever it is I feel insecure about)?
If the answer is yes then you would make a plan and execute it.
Read books about other people who had the same issue and how they overcame it, purchase products, attend workshops/seminars, research articles online, and if necessary get support through a group or from a therapist. It all depends on what the issue is. Primarily you just want to be "proactive" in finding a solution.
Thirdly - If you determine that it is NOT something that can be changed. In other words, "It is what it is". You will have to learn to "accept it" as part of you. For example a birth defect or some other physical issue.
You will probably want to join a support group as well as learn to focus on the atributes you are proud of. It also doesn't hurt to learn to utilize a little "self deprecating humor" to break the ice and acknowledge the big elephant in the room. Once that is done display your confidnece with whatever you came to discuss with them. The majority of people will get past it and hone in your personality and accept the knowledge you are offering. People respect and admire others who are confident and are able to deal with challenges in life.