So I'm a few days late to this discussion but: I'm a plotter, so I plan all major plot points out in advance, but as for where to start, I settle on a killer opening line that clearly sets the tone and mood of the book, and maybe has a little bit of foreshadowing, too. The rest falls into place for me, after that.
For example, the opening line from my WIP:
Rosemary Espinoza is still catching her breath when TĂa Carmen says “You’re going to fall in love this October.”
Sets the tone, gives the audience a pretty good idea of what to expect plot-wise, and it's a natural transition into the rest of the scene, because if my aunt told me I was going to fall in love in October I would have some questions lol. Because I love examples, here's two more from other books I've written:
It's funny how quickly dreams-come-true can dissolve into nightmares.
and
The air in Fairview Cove is much too thick to be normal.
This method works pretty well for me, but it's important you find what works for you! I personally think if you nail a killer first line, the rest will write itself. It leaves the readers with questions, which will compel them to keep reading!