You need to create a virtual switch first of all - there is probably already one there - host-manage-networking. After that you need to create a port group for your virtual machines to connect to. The port group will be assigned one or more of your physical network ports - e.g. VM Network. Once you have a port group you can edit your virtual machine and a virtual NIC or modify one if already there by selecting the port group it should be connected to. You need to decide which physical NIC(s) you want to use for Virtual Machine traffic - normally you'd use say 2 for ESXi management traffic (2 for redundancy) and 2 for VM traffic also consider whether you are using VLANs.
This might help - chapter 2.