Fiberglass is on the pricey side, so you'd want to make sure you enjoy the sport before you shell out some big $$.
I agree on the point about short, beamy boats. They are popular because of low price and good stability for a novice. They are also slow and take more effort to paddle because of their beam to length ratio.
They are fine for goofing around, but you'll want something longer and with a finer profile if you want to cruise, fish or camp out of it.
Again, your planned use will (or SHOULD) dictate they type of kayak you buy.
Try a friend's if possible, try rentals, talk to people that are already doing the sort of activity you have in mind.
The best advice you could ever get!
Been kayaking for years now and I started off with one of those beamy short recreational kayaks and soon ditched it in short order for a Ocean Kayak Prowler 13. an what a huge difference!!! Im now looking at the Trident 15 or the Hobie Adventure for my next upgrade. I mainly fish from mine though.
Dont shy away from a kayak that is narrow beam wise and feels a little on the tippy side. Just like a bike once you get used to it and dont need the training wheels you will find that you can easily stay balanced and not tip over. I have taken fish up to 100 lbs in the ocean and have yet to tip my yak.
Spring for a good paddle too, it will make a huge difference in you level of enjoyment. A good paddle will allow you to paddle longer distances with less effort and fatigue. While a carbon fiber paddle is great they are also expensive and not really needed. A good paddle in the 100 dollar range will serve you well. The 30 dollar or so aluminum paddle really leave a lot to be desired.
A good high back seat with plenty of lumbar support and a nice cushy gel seat will make extended time in the yak a lot more comfortable too. The seat is not a good place to cut corners when shopping for a yak.
The areas where you plan to use your Yak will have a profound effect on the models of Yak that will be most suitable in your area. Short beamy yaks work best on small waters. Long skinny yaks work best on big waters and long trips. Going BTB's? Look for a yak about 13-16ft with a good bit of rocker. If your fishing from it make sure you can get your rod tip past and around the front of the boat. If not on a big fish your likely to take an unplanned swim.