How do i get black shadow like this behind my text?

Layer style effect > drop shadow

A method to do this might depend whether you have the text on its own text layer.

If it is still a text later, then with Blending options (click the fx icon for Layer Style) you can add a Drop Shadow and adjust as required.

If the text has been rasterized (eg. a flattened image) then it becomes more difficult.

I tried to 'brush' the black 'drop shadow' on a new layer above the background and set the layer as 'Overlay' which worked for me with white text, but a more knowledgeable Ps user will surely give better advice.