Probably best to start with this :-
www.wandsworth.gov.uk/downloads/file/4966/choose_a_wandsworth_secondary_school_brochure
It will tell you how big the catchment areas are for each school - most of the more sought after ones have a smaller catchment are, while some of the less sought after ones accept everyone who applies (not an indication of how good the schools are, of course, just an indication of how they're regarded). It will also tell you whether religion is an issue.
You can measure the distance between your prospective home and the school using google maps or similar and see whether you're likely to be close enough to get in. Some have specialist exams (eg. in art, languages, technology) when your child is in year 6 too, which could be an additional avenue in (as long as your child would be interested in that specialism).
It will also help to look at the DfE website to assess results:-
www.education.gov.uk/schools/performance/
Or OFSTED for a more general view :-
www.gov.uk/government/organisations/ofsted
Once you've narrowed it down, come back to this website (which is a fantastic resource!) and ask for info on particular schools - there's always someone who's children are there/who know someone who is and are willing to help.
Good luck, hopefully that's helpful! 