Activity Monitor: How to know what processes to turn off?

I don't think it's entirely up to date, but most of them should be here: http://triviaware.com/macprocess/all

But do NOT cancel things you don't know for sure you don't need. Your Mac runs lots of processes to take care of itself.

Worry more about not installing things from untrustworthy sources, and making sure there's always at least 15% free space on your internal HD.

If you're having performance problems, address them directly.

Ah, in that case, watch your CPU usage.

When Activity Monitor starts, select either +All Processes+ or +Active Processes+ in the toolbar, and sort the list by the +% CPU+ column, so the highest numbers are on top, and select the CPU tab towards the bottom.

For this purpose, also select +View > Update Frequency > Less Often+ from the Menubar.

Various processes will "spike" up and down normally as they do things, then are idle, so you need to watch it for a while and get a feel for the processes that are using the most CPU, and the +% Idle+ at the bottom.

When the slowdown occurs, see if something changes significantly, especially if the +% idle+ goes to near zero. If that happens, see what process(es) are using the most CPU.

If the process(es) involved aren't familiar to you, post their names here (and/or check this out: http://triviaware.com/macprocess/all)

You may also want to select the +System Memory+ tab towards the bottom. Keep an eye on the total of Free and Inactive memory (if they're near zero, you may need more), and the +Page outs+ (if that's high, it's another indication you may need more memory).

If your Mac running OK, you don't need to turn anything off.