Ok. So you know that you need to simulate a WSN with 5 transmitters and one sink, but what else do you know about the network? Things like topology, protocols involved, energy, transmission schemes, duty cycling and etc.? Without knowing your full needs, it is near impossible to suggest something.
But, in general, you are looking at using something like NS2, NS3 or Opnet for simulating such a network. Getting instructions to simulate networks on these simulators is not difficult; you just need to search on Google, and you'll find enough resources.
You can also use OMNET++ with a framework, for example, Mixim.
They provide some helpful examples that you can use as a starting point.
As it was already said, you must have some idea about what to simulate, such as protocols and traffic type, so you can change the code and configure the simulation according to your needs.
I suggest you to try the example simulation call 2-node that provided by Qualnet. It is a good start to get use the qualnet before you design your network based on your need. please be aware qualnet simulator is licensed software so u need to join the qualnet forum for further discussion
I suggest to create a network with 6 nodes in ns-2. Make 5 nodes as the source. Attach a UDP or TCP agent with these sources. Make one node as the sink. You need to write a tcl script.