How do I get a legendary car as a newbie?

You can get it by collecting 360 secret keys at the Kart Center. 360 secret keys = 10800 batteries

You can only obtain the 'Daejangcha' (leader car) through gacha pulls after a very long time of collecting batteries (10800 batteries) or secret keys (360), or by spending money (Special Benefit for 4 months at 24,000 KRW, or Cube Pang Pang at 132,000 KRW, or about 170,000 KRW for batteries). The reason you need to pull 360 times is for the rear decoration. For carts that include rear decorations, not just the 'Daejangcha', the rear decoration is essentially the main body, so obtaining it is essential.