Question

If an organism starts from only one cell,how do we get different body parts and organs

Answer 1

Every cell has chromosomes in its nucleus.  In a human cell,
there are 26 of them. AND ... lined up on every chromosome
are thousands and thousands of GENES.  Those carry the
instructions for what kind of organ or bone or tissue or hair
this cell will be used to build, how the structure will work, how
tall you will be, what color your eyes will be, how deep your
voice will be, and what illnesses you will have. 

Every cell has the complete set of instructions in it.  In the past
several years, we have just started to be able to read them, and
extraordinary progress has been made.  There's an awful long way
still left to go.