How to get to original OS on my Mac Book Pro late 2011?

[snowshine]

My Mac Book Pro bought in Nov 2011.
Specification:
Mac Book Pro 15
2.2GHz Quad Core i7
4GB 1333MhZ DDR3 SDRAM -> upgraded by myself to 16 GB in 2015
128GB Solid State Memory
Superdrive 8x
with
AMO Radeon HD 6750M 512mb GDDR5

This machine failed to boot. First I thought it was the graphic card related problem. I posted to seek some help here last month. I could not get to work.
Left it on the side for a month.
This evening I just out of curiosity, attempted to boot holding Command + R and I was getting to a screen to select language then screen flickers and Blue screen.
Next attempt it took as far as repair but then started to flicker and blue screen.

I remembered me changing RAM to 8GB two years ago bought from crucial. Just out of curiosity removed both crucial RAMs and inserted the original RAM sticks that was shipped with the computer.
Afterwards my computer is booting normally and no crashes so far-Just an hour has passed.

My question is how could I return to as if was factory shipped.
I have installed the Mavericks OS using a usb stick in 2013.
I think this computer was shipped with Mountain Lion.

How could I return back to the original OS that was shipped with this computer. If that is not a good idea, how could I do clean reinstall of the Maveric OS that I have on a USB key.

I would very much appreciate for your guidance.
Thanks a million.

 

[Audit13]

If you have a Mavericks installation key, boot from the key, user disk utility to erase the drive, and install.

You may be able to perform an internet recovery too.

 

[snowshine]

If you have a Mavericks installation key, boot from the key, user disk utility to erase the drive, and install.

You may be able to perform an internet recovery too.

Thank you Audit13.

I wish to erase all on my SSD and want to go for this clean install of the Maveric that I downloaded and saved in 2013.
I have a fast fibre broadband connection. This computer is connected to internet through WiFi on the same network.

Would you put in here a link that I could read to perform clean OS installation.
(I must have done a clean Maveric OS installation in 2013 but completely forgotten the method0

Thanking you

 

[Audit13]

Plug in the usb mavericks key, boot the machine while holding down the option key, select usb key as the boot drive, open disk utility, select the boot drive, erase the boot drive with GUID as the partition table format and file format as journaled, close disk utility, install os.

An internet connection isn't required unless you want to use the internet to install the os that came with the machine.

 

[ferrarr]

Here is Apple's recommended procedures for restoring your Mac through Recovery mode. I also included what to do before selling your Mac.

https://support.apple.com/en-us/HT201314
https://support.apple.com/en-us/HT201065