I know many movies show this to be a easy task with lots of action packed key presses, but it's really not. Hacking into a computer is about the same as breaking into someones house. You need to know anything custom they put into the house that might allow access from the outside. Maybe a doggie door for letting the dog in or out. OR maybe they installed a new slot in the door for the mail. Has the keyhole been changed, etc. You need to have SOME information on the computer you are trying to hack, and not just OS type. You would want to know what is running on the system, how protective are they about their computer, is the computer being monitored, does it run any networking programs, and so on. After learning this, you should start looking up exploits for those programs to allow access into the system. The most common method (I've been out of the game for a few years, so about 2 years ago) was to use a buffer overflow exploit to overwrite the memory of a program and have the computer run that code. With C# out, I think it takes care of this, but some programs are still susceptible since many companies hate C# for it's platform dependency issue. Anyhow, the code you run should be able to either add a guest account, unlock the administrative account, or even activate a trojan or something similar pre installed to download a backdoor and install.
IF you are REALLY desperate and want to try the quickest method, try to do remote desktop. There is a way of using it to where the other computer is not asked for permission. By default, this automatically disabled, but if the person disables many of the UAC's on the system, it may be possible.
If all else fails, return to the net and ask again. This is not something you can write a tutorial on though. The reason hackers are called "hackers", even though by definition that is incorrect, is because it is a "respectable" title for people on the internet with skills. The reason your average Joe is NOT a computer hacker is because learning to hack is about the same as learning to skydive by jumping out of a plane without practice or lessons before hand. You either catch on quick and start learning in full force, or you wander around, accomplish nothing and give up. Any good hacker would tell you what I'm about to.
"Figure it out yourself."
You cannot enter a world that is constantly evolving when you cannot learn the material yourself by yourself.
Much of that was a ramble, and I slightly lost track of where I was going. Apologies if I hurt your feelings, but many people have asked this on these forums in the past and I have tried to teach them. I've found that it never works unless they go onto good, type some search terms, and start learning by themselves over the course of the days. Hacking is not something someone teaches you, it is something you learn. That is why there are so few hackers, and why there are so many leeches.