Two log files are going to interest us which are openhab.log and events.log
The events.log file contains what is happening on the Openhab event bus like sensors changing states, very handy for writting rules as you can see things changing in realtime.
The other file openhab.log shows the output of all bindings and addons and allows you to fault find issues with a binding.
Before asking for help on the forum, it is worth checking openhab.log for clues as to what is causing any issues as this will result in getting help much faster.
To watch the logs in realtime with a openhabian based setup use the below linux command which can be done via SSH with a program called putty from a windows or mac machine.
CTRL+C will close the stream.
You can also use SAMBA/network shares to open or copy the file directly.