Organic Chemistry, basically is defined as the study of hydrocarbons.
That on a literal note means compounds containing 'H', 'C' as their cheif elements though they may also contain sum other elements like S,N,O, ...., etc., either as their constituent part or linked to them.
The first and foremost thing to be done knowing a bit about carbon & it's nature.
'C' is the element which forms millions of chemical compounds unlike any other element.
Now the question is why only 'C' and not any other element ?
And a simple answer is there are two specific properties of carbon, responsible for this nature of carbon to form millions of compounds namely Catenation & Allotropy.