What is the formula for find the volume of an object?

You can measure the volume by filling a container (the best option would be an accurate measuring cilinder) with water. Note how much water is in the container, now immerse the object you want to measure in the water and once more note how much water is in the container. The difference between your second measurement and the first is the total volume of the object.

Another way would be to weigh the object and then calculate its volume via its density

Density=mass/volume so volume=mass/density. This method only works if your know the density of the object.

A third way would be to simply measure the dimensions of the object and mathematically calculate the volume (e.g. a cube would be length*width*height).