Question

how can u choose a gaussian surface

Answer 1

Gaussian surface can be chosen in a clever way such that you can use the simplicity of symmetry..

We first use the flux equation \[\phi e =E. ds. \cos (\theta)\]

now the gaussian surface should be such that.. E must be constant over the entire surface.. or atleast you should be able to divide the surface into different parts.. such that each part has some constant E..

in those every part, the theta value must always be same too..

Check professor Walter Lewin's Lecture on Electricity and Magnetism 8.02 Gauss theorem and applications on youtube :)