You don't need to entirely finish Arrow to start flash, but at very least watch first 2 seasons of Arrow beforehand. Then if possible, you could watch episodes side by side after since S1 of Flash ran on tv during same time as s3 of arrow.
2 other shows if you're interested are Legends of Tomorrow and Supergirl. Legends had an episode on s4 of arrow+s2 of flash to help start up the series. It also features some secondary characters from both shows.
Supergirl for most part you can watch without seeing other shows since it doesn't take place on the same "Earth" and has really only 1 crossover episode featuring 1 character from another show.

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