Interfaces with vb2005 project

Hi,
I am developing a windows project.
In one of my form I have few procedures and functions. Everything is working
great.
Now I am told to do it through interfaces, means create a interface class
and then implement it. I am little concern doing it.
1. Do we really have to create a interface when we have to implement it ONLY
in one class nowhere else.
2. Are there any performance issues when we are using interfaces methods
instead of method with not interfaces implement.

Also I want to know when is the right time to use Interfaces.

Thanks in advance,
Mike