Ask HN: Writing my first app

I've got a killer idea (!) and want to write my first app. What's the best way to do it. I'd like to optimise widest coverage for time spent initially.

Which platforms should I target. It's to be a paid app for parents (though generally info about targeting demographics vs platforms appreciated) but I'd consider free as they're are strong advertising opportunities. I'm assuming iOS is going to be best but I'd love to develop once and deploy to android as well if that's even possible. Primarily I see it being used on phones.

Primarily my closest domain experience is in webdev (PHP, HTML, JS) though I've done a tiny bit of python, C++, SmallTalk, Fortran in the past.

I don't have funds to purchase anything, I'm on Kubuntu but have Windows accessible.

Thanks for any info and advice.

[Sorry if this is a dupe of a dupe ... I did search but couldn't find "App Dev 101" - this is probably the closest https://news.ycombinator.com/item?id=4731469 (the link should be http://www.codelearn.org/rails-tutorials) but it has no info about implementation on a device, distribution, getting in to app-stores and such.]