Login - Screen Flashes and Returns to Login Screen - Terminal Explanation I found this question Login - Screen Flashes and Returns to Login Screen - Tried Everything Possible, which is exactly like the problem I'm having.
But the answer confuses me....

There is one troubleshooting step that OS X shares with the classic Mac OS, removing preference files. But on OS X there are TWO folders:
/Library/Preferences and ~/Library/Preferences
--The latter being in your user/home folder--
Since the issue doesn't exist under another logon it is specific to that user folder. SO there is something specific to that user that is causing the problem. This takes the /Library/Preferences folder out of consideration for troubleshooting preferences. The first thing I would do is use a utility like Onyx to verify and clean the system. This includes verifying preferences files. After that rename the ~/Library/Preferences folder to something like
~/Library/Preferences-old
You'll probably have to do that in Terminal and use sudo as those files are owned by another user on the system. then create a new ~/Library/Preferences folder and reboot.
If you can now log into that account it is a simple (HAH!) matter of putting preferences files in the new Preferences folder and rebooting till you find which one it is.
No guarantee, though, that it is a preference file, it could be a startup item. So do the same thing with the ~/Library/LaunchAgents folder.
The other thing to consider is that it might be easier to just create a new user folder. Copy the things you need out of the old user folder and delete the old user. That could save you a lot of troubleshooting time. It would be my preferred way of doing it (after running Onyx) as I care less about what happened than just finding a fix that gets me up and running more quickly.

How would I go about doing this? I have terminal open, what do I type?
My computer is a hackintosh, I did not build it. This problem just happened suddenly.
To rename the folders you listed in Terminal you would use the move command or "mv". Like this: (I will use an account called "userx" for this example)

sudo mv /Users/userx/Library/Preferences /Users/userx/Library/PreferencesOLD
sudo mv /Users/userx/Library/LaunchAgents /Users/userx/Library/LaunchAgentsOLD


To create directories in Terminal use "mkdir"

sudo mkdir /Users/userx/Library/Preferences


Lastly, the step they skip is allowing "userx" to use the new directory you need to make userx the owner of any new folders you create in this manner.

sudo chown userx /Users/userx/Library/Preferences