Kashyap,
That’s an extremely broad question… have you looked at any sketching tutorials on YouTube?
Spencer Nugent… look for the one called “how to sketch anything,” which is probably most relevant to your question: https://www.youtube.com/playlist?list=PLjZ8ePITBc0KXHkDGMOHhumpTqPv3TA8r
Michael DiTullo: https://www.youtube.com/channel/UCyLFNm0OPvpG2CK_Zn9bpeQ
idcreatures: https://www.youtube.com/playlist?list=PL7EBF209E25F1624E
There are a ton of others out there, too. I also recommend following sketchers on Instagram (including Michael and Spencer) to keep a steady influx of inspiration. There are also some good sketching books out there, but I find videos to be much more helpful. One that comes to mind is Koos Eissen’s “Sketching: the basics”.