How you start your story depends on the type of story and the genre. However, the main plot should begin with the first plot point - inciting incident - the thing that propels the protagonist into their struggle to gain their goal. Never start the novel with backstory.

If you're writing a mystery or a detective novel for instance, then that inciting incident, e.g, the finding of a murder victim, should come somewhere in the first two chapters. It is still worth introducing your character in their 'ordinary world'. Readers want to know who the story is about and want to engage with the character so he/she should be shown doing something interesting i.e. something unusual or challenging for them before they are thrown headlong into the plot by the inciting incident.