If you want to be absolutely certain your child will get into a good secondary, you will need to find a good comprehensive and move right next to it, preferably within .5 to .75 miles of your choice. (Catchments expand and contract, but 0.5 will usually do the trick and I'm allowing for the passing of a few years before your DDs apply - exceptions for the insanely sought after like Fortismere). Some good comps have catchments of about 1 mile; you can get this info from council websites. Also be aware that while most award based on distance as a straight line, some have more complex systems.
You can still apply for the selective schools, but don't bother moving right next door, as they do not award on distance. If you want to go for the selectives, have a teacher you trust give you a realistic assessment of your child's chances and then plan on some tutoring, maybe a lot of it.
To find a good comp, first look at Ofsted and league tables to narrow it down - in particular look at % acheiving Eng Bacc - then visit, visit, visit. Because a good school is not all about GCSE results, of course.
Try to find a comp that has had consistently good reviews over a number of years. Many tend to rise and fall with a particular headteacher, who obviously might leave. You'll hear parents say things like, oh we're going for school XX and wouldn't even have considered it 3 years ago, but now it's great. I'd want to know WHY it's suddenly great and whether the improvement looks sustainable. If you visit and get a great feeling, that's a really good sign.
Also be aware that many secondaries have either just become academies or will do in the near future. How this will affect them is anyone's guess.
It's a fucking headache, frankly, and all the best of luck to you.