Reset your Mac
Reset your Mac options
Since many people coming here use the term "reset" rather loosely without specifics why, I've created this User Tip to show the three options:
A: Wipe OS X, programs and all files off the machine and reinstall what you want from there.
Used for fixing drive structure issues, malware issues or giving up ownership of the machine or setting a used machine that wasn't properly restored, to you personally. (see A below)
B: Replace just OS X with a new copy, leaving files and most programs alone (should backup if possible). Used for fixing OS X based issues only for the current OS X operating system owner, will not change ownership. See #8 here.
..Step by Step to fix your Mac
C: Create a new user account and transferring wanted files to that account and deleting the old one, leaves OS X and programs alone that are in the Applications folder, programs stored in the user account are deleted. Used for fixing in User account issues.
Combine two+ Users Accounts on the same machine
Mac's do NOT come with System Restore options like Windows has, however if a TimeMachine or bootable clone drive was made previously the entire machine or files can be restored since the last update of either.
Backup or perform user(s) files data recovery first if possible.
You need to have two copies of your wanted users files off the machine to separate hardware, unmounting drive (drag icon to Trash to Eject) and physically disconnecting.
If your having software or drive problems I highly recommend also making a emergency copy of just your users files to another regular external drive as you might need to only restore those and everything else will be reinstalled fresh from original sources.
Most commonly used backup methods Drives, partitions, formatting w/Mac's + PC's
If your boot drive is not booting: .Create a data recovery/undelete external boot drive
For a deeper understanding about the hidden partitions on a OS X boot drive:
Erase, formatting, OS X installs on Mac's
A: is broken into three parts.
1: Giving up ownership of the machine or making a machine your own.
You have to install the operating system version that came with the machine from the factory or 10.6 (earliest you can).
The upgraded operating system versions are tied to a AppleID and the next owner (or you) can't perform updates.
You do this by holding command(⌘) option/alt r keys down (using a wired or built in keyboard) while on a fast Internet connection (Ethernet cable to the router preferred) and booting the computer into Internet Recovery.
Disk Utility is used to erase the disk0 (if a boot hard drive, move the slider one spot to the right) and then Quit when finished and OS X can be reinstalled. After completion you hold the power button down to shut down and the machine will boot to the "Welcome to Mac" video for the new owner.
If OS X 10.6 (or earlier came with your machine from the factory) or Internet Recovery doesn't work on your machine, then use the 10.6 install disks instead.
How to erase and install Snow Leopard 10.6
Once OS X is installed the machine should reboot to the "Welcome to Mac" video which you press and hold the power button down to force a hardware shutdown for the next owner to boot into and setup.
If your making the machine your own: Then you log into AppStore with your AppleID and password and can upgrade the operating system version there.
Note: OS X 10.6.x must be Software Updated to 10.6.8 via the Apple Menu to install AppStore, then it's under the Apple Menu to use.
Some Mac's can be upgraded to later OS X versions that Apple doesn't advertise on AppStore, you can research the latest OS X version for your machine using the free MacTracker and by calling Apple directly and ordering the appropriate OS X version to download link.
For more assistance for making your machine your own, complicated machine id issues/not booting etc:
How to reformat a used Mac Changing ownership of a used Mac
2: Not giving up ownership of the machine. Fixes issues in the MacintoshHD partition.
Reinstalls the current upgraded OS X operating system version or the factory one if it wasn't upgraded. AppleID required.
You do this by holding command(⌘) r keys down (using a wired or built in keyboard) while on a fast Internet connection (Ethernet cable to the router preferred) and booting the computer into RecoveryHD which is a hidden partition on your boot drive.
Disk Utility is used to erase the MacintoshHD partition (if a boot hard drive, move the slider one spot to the right) and then Quit when finished and OS X can be reinstalled.
After completion you quit to reboot and the "Welcome to Mac" appears for new setup which you can transfer or restore from a bootable clone or TimeMachine backup or create a new account and transfer files manually from a regular external backup drive into the fresh account so it's free of issues.
3: Major drive issues requiring a complete reformat or on a brand new drive.
Same as 1: procedure above using Internet Recovery or the 10.6 boot disks.
You select "disk0" or the entire drive makers name on the left in Disk Utility as you need to catch the entire drive to erase/format hidden partitions of EFI (and RecoveryHD)
Select the Partition Tap and the partition is 1 (or more) click the box and Option: GUID and the format of the partition OS X is installed into is OS X Extended Journaled.
OS X cannot be installed without a GUID partition table and OS X Extended Journaled as the format.
After OS X install is completed you quit to reboot and the "Welcome to Mac" appears for new setup which you can log into AppStore, option click on Purchases and should be able to redownload the OS X upgraded version again if you have upgraded before.
Some Mac's can be upgraded to later OS X versions that Apple doesn't advertise on AppStore, you can research the latest OS X version for your machine using the free MacTracker and by calling Apple directly and ordering the appropriate OS X version to download link.
Once the OS X upgrade is completed, then you can transfer or restore from a bootable clone or TimeMachine backup or create a new account and transfer files manually from a regular external backup drive into the fresh account so it's free of issues.
Also you can use Migration Assistant in the Applications/Utilities folder and the C: method above to transfer files between accounts.
WARNING! Third party user installed SSD's need TRIM support enabled using third party software.