You should know hacking =/= Crime. Hacking is more like a lifestyle. Anything can be hacked no matter if it is your fridge light or a multimillion company. But let us take the process of hacking a (pc) "personal computer". Which belongs to your Girlfriend.
Maybe you would expect a modern operating system ("win7,8,10", "OSX ",) not to be vulnerable at all. But that is not True, there a nearly unlimited ways to break into a computer. But your set of possibilities depends on your range of knowledge.
There are preconfigured tools which can be used to get access to the computer of your victim. One of them is the "Metasploit the world’s most used penetration testing framework". Link= https://www.metasploit.com/ this tool can be used to find Vunablilities in a system, exploit those, get persistent access, hide your tracks, and many many more. The chance to get vulnerable one depends on factors like: "release date of the software used / os/browser", "the skill to perform social engineering (for example to search a reason for a person to visit your website)", "the technical knowledge of the victim. And again many many more factors which I'm not able to count. There were instances of time where nearly every Android device could be hacked just by clicking on a link. This vulnerability/bug was called "Stagefright"
link:https://en.wikipedia.org/wiki/Stagefright_(bug). There is also one recent called "Blueborne" which could give an attacker full access to a victims device by just staying next to her/him for a couple of seconds. There are many more vulnerabilities out there, many of those are not found yet. After 5 minutes of google search, you will find a free and automatic performing tool for your vulnerability of choice. And in most of the times, local access to the victim's computer == direct access to his data. In most of the cases, you will be having access to most of the device's functions. For example, controlling/spying through his camera, dumping his messages, spoof his number (also works without access), downloading his/her files /uploading a file, destroy his device, encrypt his/her data .... more....).
And don't forget that this might not be allowed in your country.! "or even get you in jail"