When you go into your actions window, under the skills tab set one of them to the fishing skill. That button can be copied to your hotbars and it will activate fishing. Of course you need to have the pole in your primary slot, have bait, and be near water.
You can also create a social hot key to cast, pause, and attempt and /autoinventory command to make things easier.
If I recall correctly, it will look something like:
/pause 40, /activate [whichever number is your fishing skill 1-6]
Hopefully that was correct. It's been a while.