Hi,
Firstly, I would say it is good to be playing adult cricket -it should hasten your learning about the game. I try to go through a game thinking about what I would do, if I were captain. Linked to this, is to study each batsman to try to identify technical weaknesses (eg what typ eof ball do they hit in the air/ mistime etc). This will help you to bowl intelligently.
I don't think there is an easy way to increase speed. The basics are to ensure a good bowling action; maximising the distance your bowling arm travels before release. Don't strain for too much speed, too early in a spell - try to make sure you are fully loose and warm, find your rhythm, get your run up honed and find your direction. After a couple of overs you can start to push the ball through, quicker.
In the nets you may want to try increasing your run up speed, or jumping a little higher during the gather, to see if they are technical changes that work for you, in increasing speed.
Good luck,
David