I am planning to buy a bicycle; can u advise what to look out for when buying?

Firstly it depends on what you want to use the bicycle for. Would it be for work, grocery, marathon or just for some casual runs around the neighborhood park. Based on that, look a few brands and their prices. Get an idea of the price and set aside a budget. Pick a few models around that budget and look at customer reviews, video reviews are more helpful than written reviews in my opinion.