As I know evolution comes bit by bit mutation by mutation
How sex evolved which requires a major change in at least two individuals one to become male and one to become female ?
When that happened as we can see sex in all animals and most plants?
Sex (depending on how you define it), evolved very early (more than a billion years ago). Even species like yeast have a type of "sex" (see below).
Also, this doesn't mean there is necessarily a big difference between the two sexes.
The yeast Saccharomyces cerevisiae, for example, also has two "sexes", called mating types. One is called alpha and the other is called a. They can only mate if they have different mating types, so it is analogous in a way to mammalian sex. However, the genetic difference between these two types is tiny. There's a specific location in the genome that's called the MAT locus. This location can have either an alpha sequence or an a sequence, an that will determine the sex. The rest of the genome will be the same between sexes.
So it really doesn't seem like it is a huge evolutionary step to go from no sex to some simple version of sex.
See more details about yeast mating here.
Also see this wikipedia article on the evolution of sexual reproduction.
The oldest evidence we have of sex comes from the fossil record which shows it was present more than one billion years ago - it could be even longer (it would be naive to think we have found all fossils or that all organisms that ever existed became fossilized). Sexual reproduction introduces potential benefits because it allows recombination meaning useful gene combinations are created more regularly and deleterious mutations are lost more efficiently (see Muller's Ratchet).
It is thought that sexual reproduction first occurred in single celled eukaryotes. In bacteria it has been shown that stress can induce the process of transformation. Transformation is a sex-like process in bacteria where one bacterial cell takes DNA from a donor bacterial cell and incorporates it in to it's own cell, it could help to repair DNA damage caused by stress. Transformation could represent one early form of sex.
It is difficult to put a timeline on it because it can only be seen from fossil data. But from early primitive haploid organisms we would need the evolution of diploidy, some sex-determining mechanisms, anisogamy, and reproductive physiology for basic recognisable sexes. From transformation you could get early eukaryotes that take up extra copies of genes from donors, leading to primitive diploidy.
To put down some unsupported thoughts (because this question is very hard to answer - there is no really firm consensus)... Once diploidy (having more than one copy of a section of DNA) is established it may be possible to form some gamete type functions, and anisogamy, where donors can more actively package, send, and receive large sections of DNA. From there anisogamy could encourage the specialization of gamete cells more similar to what we are familiar with (eggs and sperm). One likely next step would be through hermaphroditism - the ability to be produce both gamete types simultaneously or sequentially. Then some individuals would specialize in being one or the other (sex-determination). Continued evolution would lead to more distinct male and female phenotypes.