i want to hack things and i don’t like getting zapped and setting off alarms 
5+ computers skill should be enough to hack most things.
All of it. But it mostly depends on what you are trying to hack. about 4-6 is good for banks, sewer samples, mines maybe lab log computers anything above that is good for all the military and other lab stuff.
You’ll need at least 10 to hack tank drones with no chance of failure.
If you got no access to books, hacking the computers at labs should train computers steadily. Safely if you got protection against electricity.
For labs, the basic turret/console combo needs about a 3 to be mostly reliable-ish and relatively safe (I think they just shut down if you fail), but don’t go for the CBMs until you’ve got at least 6 - one their failure results is to drop secubots in with you.
The analyzer is the best way to train - get something to protect from shocks, have a way to deal with manhacks, and you’re golden - they never shut down and never drop anything worse than manhacks. (Great way to farm plutonium, too, if your computer skill is low.)
The observation rooms (the ones with the brute/migo in the middle) don’t drop anything, but they can shock you or shut down. Same with the entry consoles for barracks and prisoner containment.
Of course, the easiest way to train is to find an eye bot and control laptop the resulting bots - effectively limitless computer training.