GMAT&MBA query

People, who gone through the journey of GMAT, tier-1 MBA college India or US, how do you assess whether or not to join a early stage startup right after college, does it affect chances of getting in? keeping in mind I don't come from engineering background or tier-1 college.

Also, where did you study for GMAT? I gave it once after prepping for a year to score around 650. Any recommendations?