We used the local council website to create a shortlist of schools we would have a chance of getting into (important if you live in an area that is heavily catchment based) and went from there. We moved this time last year (though to an area we knew well and have a long family history in, so weren't enrirely starting from scratch) and DD started in Sept.
Councils normally publish the PAN (what the maximum intake will be, e.g. a PAN of 60 would usually be 2 classes), along with info from the previous year, e.g. how many applications each school had, how far away the furthest offer was and which of the admission criteria they got down to before they stopped giving offers.
Aside from the obvious sources of information online (ofsted- ignore the single word judgements, take a look at the detail; school website; gov website etc), I would say that visiting is really important- I completely discounted one school that I might have put down after visiting it. I also wouldn't underestimate how important a good headteacher is- they set the tone for the whole school for years to come- along with those things you can't pick up on without visiting (things that friends across the country have mentioned that turned them off in schools on various places: no artwork on the walls; a lot of mentions of 'discipline', miserable looking children in 'every class'; an uncontrolled class where the teacher had no control; a lack of 'joy' from the kids; a headteacher who spent the time talking about how the school had no money; a rude and dismissive head; a head who focused on why not to go to another; more popular school rather than talking about why to go to theirs).
Things that the same friends were positively surprised by included: how much the PTA has helped raise, and how it has been really well invested I to things to help the kids; headteachers who just inspire confidence; excellent, approachable SEND and pastoral staff; 'good vibes'; pride in the children's work, with displays of work at all levels; a headteacher who made it very clear that no question was off the table and that she was proud of her school and the children there; a very happy student body; a good sense of community and belonging.
It's also worth looking at local Facebook groups as well- if you discount the people who are persistently angry for one reason or another, or those that hold a grudge, they can be a source of very useful info.
Will you have relocated (or exchanged, if buying- so you can provide evidence) by the Jan application deadline? If you miss that, things can get complex. Though there is usually a late application deadline, choices start to narrow after that, particularly for popular schools.