I think you misunderstand how Wifi works. Wifi is a method to provide network connectivity for a device, much like the 3G phone network. You can use TCP/IP sockets on top of it, but you don't use sockets "to connect to a WLAN" - Android itself does that, you just have to turn it on.
By "then" I assume you mean "them" - if you're supposed to connect to "them", then "they" should tell you which port to use. Unless you intend to use some standard TCP/IP protocol, most of which have established port numbers: List_of_TCP_and_UDP_port_numbers
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