Who was the first person to suggest the idea of atoms?

Several ancient philosophers such as Anaximander, Democritus, and Anaxagoras had concepts of the atom. Anaximander believed that matter combined with motion made a unit; Anaxagoras believed that each and every substance had its own kind of particle (which he called 'nucleus'), there was an infinite number of them, and they could be divided endlessly. However, the first to come up with the concept of the actual atom was Democritus. He believed nature was comprised of the ceaseless motion of small, material, indivisible and eternal particles which he called atoms ('atomos' from the Greek for 'indivisible')
Hope that helps. :) -Erin

Leucippus is the first person to suggest the idea of atoms.

https://en.wikipedia.org/wiki/Atom#History