Help a homie out: Need a bicycle for my weight loss journey

Hey Makka! Just kicked off my weight loss journey, aiming to drop from 140 to 70. Been hitting the gym for two weeks doing muscle training. My gym bros suggested I bike to the gym for cardio since the gym doesn’t have much—just a treadmill and an ab machine I can’t fit into yet. Thinking of picking up a second-hand bicycle in Chennai. Looking for something cheap and good. Any tips on where to get one, what type to go for, and what prices I should expect?