Hello Anony 
And a very warm welcome to the site.
The question that you have asked is not uncommon and a few of us have constructed our own hulls out of fiberglass so I would expect that you will have all of your questions answered.
To make a mould you must first produce a "plug" which is an exact size and detailed version of whatever you wish to build, in the case of a boat hull you can make the plug out of wood, plaster or use the hull of an existing model.
The surface of the plug must be perfectly flat, smooth, even, grease free and clean as every blemish will be picked up on the mold and subsiquent casting.
Once you are happy that the plug is ready then you can begin to cast the mold over the top of the plug by first applying a release agent then gel coat and finally the matting.
When set you will have an "inside out hull" with the smooth side on the interior. This is your mold.
I have over simplified the process as it is frought with problem concerning air temp, humidity, mixing and setting times……………….and so on.
There are some very good books on the subject and I would suggest that you visit your local libiary to check some out.
Hope this is of some help 
Sorry for the spelling but the checker is not working 
Paul