How to go about building additional income streams?

I am a graduate from a tier-1 engineering college and will be starting out with a job in a few months at a financial services MnC in an analyst role. I am looking to build some income streams apart from my salary over the next few years and wanted to gain some freelancing skills to build an online business but am confused about where and how to start. I've seen people working on software projects and stuff like digital marketing but can't really picture myself in similar roles. I'm not a techie, my interest lies mainly in finance and business but I'm fine with exploring and venturing into something new as well. Would be great if someone who has done it in the past can please guide as to what can be some promising skills to gain and how to go about it.