Where to start reading arrow and the flash comics?

For Flash, you can start reading from the New 52 reboot series, it's just called The Flash. I'd recommend you start reading it in trade paperbacks (small collections of issues in single books; each book contains seven issues) starting with Volume 1, which is called Move Forward (just search Amazon for "Flash Move Forward").

I haven't read any of the New 52 Green Arrow, but from what I've heard it's not very good. You can give it a go if you want (again just start at volume 1 of the trade paperbacks), but otherwise I'd recommend reading the stories Green Arrow: The Longbow Hunters and Green Arrow: Quiver.

After that just check online and in your local comic shop for standalone Green Arrow stories.

The problem is that the comics and the TV shows are pretty different. On top of that, DC rebooted their entire line a few years ago so it's still a mess.

For Green Arrow, the graphic novel The Longbow Hunters is one of the definitive stories.

Flash: The New 52 series by writers/artists Francis Manapul and Brian Buccellato is a good introduction to Barry Allen and the rest of the Flash villains, known as The Rogues.

Start at Volume 1: Move Forward.

Green Arrow: The New 52 series by writer Jeff Lemire and artist Andrea Sorrentino follows the tone of the current TV series, but storywise it is different.

Start at Volume 4: The Kill Machine. (AVOID VOLUMES 1-3)

The show Arrow is based on the character Green Arrow. Both Green Arrow and The Flash have ongoing comic series. They rebooted them just three years ago so you could start from the beginning and not have to collect a whole lot of comics. Just be aware that the show isn't a direct adaptation of the comics but more inspired by them.