I always manage my novels by breaking it into 5 sections. The 1st fifth is intro and initiating incedent, the middle 3/5ths are rising action, and the last 5th is "the final action" character and action climax. Sometimes I tag a resolution on the end. So, if the page length is to be 350 pages, then each of these sections will be 70 pages long.
So, in this novel I have 70 pages in which to tell the reader who, what, when, where, why.
To introduce the character, you begin writing about the character's day to day life. Steven King called this the "steady state." You need to show the reader what "normal is" before showing them what "abnormal" is. In the first Harry Potter, we learned that he lived under the stairs, was not considered a real "son," had wishes and dreams about being part of a "real" family, which was granted later on when he joined the Witch community. There was a flashback to the day Harry was left on the doorstep. I believe we also learned that he was able to communicate with snakes. The inciting incident was the letter from Hogwarts. This was all "needed" information which could have been related in a paragraph. The point of the novel is "showing" so you can't tell people he can talk to snakes, you need to devise a scene in which he talks to snakes-- which was the whole point of that Zoo scene.
Extrapolating from that, you need to write a list of the things that the reader needs to know about your character in order to "suspend disbelief." If your character can fly, you need to create a scene showing that character flying, or learning about the power of flight. If your character is an unwed mother barely making it on the dole, you need to go through what it is like to live like that. In other words, you need to devise scenes that "show" what you need to tell the reader about the character.
The Genre often governs how you start the work. A techno-thriller would have you pacing a large explosion in Afganistan right at the beginning, with no explanation, where as a Suspense thriller might like to keep that for later, so you need to know the conventions of your genre. Some genres begin quite subtly, with people chatting in a park, others begin with a large car wreck.