I'm about to watch the last episode of season 2 of Arrow and I want to know when to start up on The Flash, since they aired at the same time. Arrow is 2 seasons ahead of The Flash, correct?
Ideally, you should watch them in the order they aired. That is, if you want to get the most coherent story from both shows, you should watch Arrow up through the end of Season 2, then watch alternate episodes of The Flash Season 1 and Arrow Season 3. There is only one time when the shows seem to be significantly out of sync, and it won't make a whole ton of difference. In particular, watching in this order is the easiest way to guarantee not to spoil anything from either show by watching the other.
The second season of Arrow contains what is essentially a two-part pilot episode for The Flash, where a mundane Barry Allen comes to Starling City as a forensics expert; the end of that Arrow episode sets up the first episode of The Flash.
The first episodes of The Flash Season 1, and Arrow Season 3, take place at the same time, and there are cross-over cameos at the end of those episodes. About halfway through the season, there is a true crossover, starting with The Flash and finishing in the following day's Arrow.
But make sure to read the full answer, since it offers alternative viewing orders.