Praise be to Allaah.
There is a hadeeth which suggests that Soorah Yaa-Seen is the heart of the Qur’aan, but this is a weak hadeeth (see Silsilat al-Ahaadeeth al-Da’eefah wa’l-Mawdoo’ah by al-Albaani, hadeeth no. 169). There is no doubt that Soorah Yaa-Seen is a great and important soorah which contains moving stories and eloquent lessons, but there is no proof that it was described as the heart of the Qur’aan.
The fact that you are asking such a question at the age of only thirteen is something which deserves praise and encouragement. May Allaah help and guide you.
Sheikh Muhammed Salih Al-Munajjid