Order of a reaction is decided by

Part: 1
The order of reaction can be defined as the relationship between the rate of a chemical reaction and the concentration of the species taking part in the chemical reaction.
Part: 2
In chemical reaction $\mathrm{aA}+\mathrm{bB}\rightleftharpoons \mathrm{xX}+\mathrm{yY}$,
Order of reaction is $\mathrm{r}=\mathrm{k}\left[\mathrm{A}{]}^{\mathrm{a}}\right[\mathrm{B}{]}^{\mathrm{b}}$
Part: 3
From the mathematical relationship, we can conclude that the order of reaction depends on the concentration of the reactants.
The slowest step of the mechanism of a chemical reaction is otherwise known as the rate-determining step.
And to determine the rate-determining step we need to know about the mechanism of the reaction.
Since, the relationship is independent of temperature, pressure, and molecularity hence, the order of reaction does not depend upon these factors.