step 1: get yourself on of these guys. it cuts thru fiberglass like butter and it doesnt make a huge itchy mess like a cutting wheel. harbor freight has em as well if you're lookin to save a buck.

cut out that rotten floor and if the 1/4 stringers are rotten they gotta go too... get the hull CLEAN, spotless. an angle grinder with a wire wheel makes short work of scrubbing gunk and grime off of fiberglass.
when you're all clean then its time to put in some stringers and a floor (if you have extra time, slap in a bulkhead i noticed a HUUUGE difference in deck strength and ride quality after doing one.. might as well while you have it all apart!) floors are nothing more than some 3/4" plywood but stringers you need some good quality, straight kiln dried douglas fir. as far as making it all stick to the hull, if you're a noob with fiberglass west marine makes a great easy to use epoxy resin called "west tec" and its 1 pump resin to 1 pump hardener. its pretty much impossible to screw up and it'll adhere to almost anything, as long as its clean.
take a peek at my thread of my current float or my old float for some visual aids.
http://www.socaljetboats.com/projects/21-bahner-io-project/http://www.socaljetboats.com/projects/78-tahiti-project/msg180336/#msg180336good luck and dont forget to start a build thread in the projects section and post up lots of pictures. we love pictures!