STEALING AND CONSEQUENCES OF SUCH ACTIONS
There are two ways to steal. You can steal items from safes, lockers and other stashes using your lockpicking skill or you can steal directly from other characters. Pickpocketing is far more dangerous, because if you didn't develop your skills to high values there's a high possibility of getting caught. The only advantage of pickpocketing is that most NPC's carry valuable items or keys which will grant you access to their homes.
If you plan on stealing from the environment, the most important thing to remember about is that you aren't seen by anyone. This applies to opening locks and taking items from your surroundings. If you're caught you'll lose all the items you've taken and if it was a neutral character you may engage yourself in a firefight. The most extreme outcome is that you're banished from a city. Thankfully this a rare occurence and it lasts only a few days.
Each time you steal something (it doesn't really matter how) you'll lose a few karma points. The differences are very small for single objects, but if you keep up with your dirty deeds you may end up being a very bad character. Lower karma usually makes your survival in the wasteland more difficult, but it's your choice. If you want to be a bad boy there's nothing in your way to pursue such goals.
OPENING LOCKS AND BREAKING INTO COMPUTER SYSTEMS
You can find out easily whether you can collect an object from your environment without any penalties or not. If the name of the object is displayed in red then it means you must steal it and you'll lose karma if you do that.
Each time you plan on opening a lock you should save your current progress, because you can brake the lock if you press the wrong button or you can lose a lot of bobby pins if you're dealing with a very sophisticated mechanism. Thankfully it's much easier when you're dealing with computer terminals. Remember that you can always quit before making a final guess of the password. When you hack into the terminal again you'll notice that you've been given a few tries. If you brake a lock or disable a terminal by mistake you can always choose perks which will help you in accessing these objects again, but it's a waste of perk points.