You can check from the wiki link here, that should be handy to perform once you have the network set up. There is Network Analyzer setting which can help you to perform the task. Obviously you have answer here but in case you need a manual to see how to do look at the link.
Also @Jean-Karim Heriche made a similar reply few weeks back about selecting the components from network. Take a look at this thread.
I would request a bit , please do not think that I am being rude. But just that we already have answer for this question , creating same thread makes it redundant. So before posting if you can try and see threads in Biostars with tags like cytoscape and network to maximize the usage of the blog to one's benefit. It will be handy for any OP posting here and be more targeted in their query. Just a request.
In cytoscape 3.x there's a button in the menu bar at the top called 'New Network From Selection' (hover the mouse over it to see the name). As the name implies, it creates a network out of selected parts of a network.