My MacBook Air keeps on turning off and on during the startup?

I have tried to open the Macbook air in safe mode by holding the left shift key when the startup tone is played untiil the apple logo and spinnig wheel comes up but after leaving for around 10 mins nothing happens. I also tried pressing the cmd button and the s key (as advised by a friend of mine ) then i was told to type certain commands into it but then it just shuts down however it didnt do this up until a week ago because then it was just starting up and staying on the loading screen for an ertanity please can you help me solve my problem and in as amatuer language as possible as i am a begginner with a Mac. Any advice will be helpfull. Thanks a lot Clyne