You can use a frequency domain power or profile monitor to record the transmission spectrum. To see a plot of the result, after the simulation has been run, you can right-click on the monitor and visualize the “T” result which is the transmission.
By default, the simulation mesh is automatically generated based on the mesh accuracy setting in the FDTD simulation region’s “Mesh settings” tab. However you can also add a mesh override region over regions where you want to specify a finer mesh. It’s best to place the mesh override only where you want more resolution since setting a finer mesh over the whole simulation region would require more memory and running time. More information about setting up the mesh can be found here:
I would definitely recommend to start by going through one of the Getting Started tutorials, and in particular, the ring resonator tutorial here:
The tutorial includes step-by-step instructions for setting up, running, and result analysis.
Or check the anti-reflection coating example in the following video for a short demo showing the setup of a simulation file: