If you check the surfing lessons section you will see a ton of helpful topics there to search through - but here are some thoughts in short.
1. you may want to rent a board at least once before purchasing just to get the feel of surfing and riding a board, see what its like before deciding, its a big expense to purchase a board.
2. many people learning have great luck buying a bic or nsp board around 7'6" to 8' or so. these boards are made out of a material that is much more durable than a traditional fiberglass board so you will not need to worry as much about damaging it. these boards are good entry level boards for that reason. they dont ride quite the same as a fiberglass board, but also will not damage and have a high re-sale value.
3. if you want to get a "real" fiberglass board, i wouldn't reccomend a full fledged longboard, as paddling may be tiresome and discouraging, but try going for a fun board or mini-mal, which is the same shape as a long board (very eggy shaped, rounded at the nose and tail, and wide), wide, around 20-22" and at least 2.5" thick so it will have lots of float which will make paddling and popping up easier.
hope those tips helped a little bit. let me know if you have any more questions after checkingf out the surfing lessons section.