Try to buy a trusted brand, Donot get trap for cheap mobiles due to their attractive low cost offers.
Should gather knowledge about their specifications from different websites.
RAM should be good ( atleast 1.5 GB )
Should consider a good camera quality, check camera features during dark hours.
Should have a good screen size ( atleast 4.5 inches )
check for interest free EMI options so that you should not compromise with a good quality phone due to a marginal higher cost.
Better to take reviews before purchasing a mobile phone.
Its battery back up and quality of battery used should be good and a pre check by observing customers reviews.
if buying online , then try to have from a trusted e commerce site.
Try to have a phone which is light in weight but it shouldn’t be too slim or too heavy.
Always ask for mobile phone accessories available with the mobile phone ( free of cost mainly )