Shivam dayal,
Is your pond a small, man-made backyard pond or is it a large natural water basin?
If it is just a small backyard pond, then a good 25-30 foot tape measure is fine.
If it is really large (like 1/4 acre or better) then you'll probably want to stake out a rectangle on the ground to encompass the pond and draw it out on a piece of graphing paper.
Scale the squares on the graph paper and use a 1/4" rope to measure across the pond and from the edges of the rectangle to the pond's edges. Then you can sketch it all out
on the paper and create a scaled map.
For depth, use the rope and a weight and a small boat or raft. Measure from the edges of the rectangle to the points you take the depth readings and take many readings so that you can map the bottom contour across the entire pond.
You can then divide the contour map of the pond on the graph paper into small sections and use geometry and trig math functions to calculate the water volume to a fair estimate.
Catfishnut