Fritz the Glassblower teaches people how to make glass on Entrana. He will also buy molten glass from players for 20 coins per piece (25 coins if players are wearing the ring of charos (a)). It is not advisable to sell glass to him as on the Grand Exchange it can sell for around 122 coins each, which is much more than he will pay.
He will give you a bucket to get you started and will tell you where to get a glassblowing pipe. He will also tell you where to find seaweed and sand.
He has also written a book which you can find in the bookshelf of your player-owned house.