The hull you have produced will be the plug.
It's not a difficult task but it is messy and can be expensive. (be sure to use a face mask and cover up well the glass strange makes you itch for weeks, and you don't want the partials on your lungs!)
Check out Neils life boat build, hes built the mould from scratch using a'plug' its probably best to see how its done rather then trying to explain it.http://www.radiocontrolmodelboats.com/t721-clyde-class-70-lifeboat-build
You will needs to get the plug to the finished state as any imperfections will show up on your mould and all mouldings there after. You will need to purchase a ton of release agent and use several coats (3 or 4) typically ont he plug to help it releases after the mould have been created, you will want a lot of resin/harderner, glass fiber strand matting, and gel coat.
Not to be a downer but casting a springer in GRP is going to cost far more then using ply, not to put you off haveing a go, but concider the cost you can buy a GRP springer hull for £20 from here http://www.modelsbydesign.co.uk/model_boats.aspx
(24th listing from the bottom)
Or you can just make from ply and seal it, once sealed and painted will take far more damage then a GPR hull which has a tendency to crack after a few knocks.