Hey! Thanks for the question - I love anything related to mecha so it's a pleasure answering that.
The first step to drawing mecha is to consume media featuring the aesthetic you like. There's so much out there - are you into the "super robot" aesthetic (think Transformers, or Megaman), or are you into more grounded "real robots" (Gundam, Armored Core, etc.). Look into cyberpunk, dieselpunk, modern machinery, etc. The inspiration is all there, but find the ones you REALLY like!
The second thing is to notice, copy and integrate the design language that goes into mechanical things. You'll notice any sharp edges are very rare. Sharp corners are very rare. Rounded features obey specific rules. When is there symmetry? When is that symmetry broken? Learn about bevels, pockets, etc. Essentially all the shape features that go into making a mechanical object in real life. This is done through observation and copying existing mechanical things.
Finally, after careful observation and copying of mechanical stuff (whether it's a sci-fi franchise or an industrial excavator, whatever suits your chosen aesthetic) you can integrate, remix, and kit-bash the information into any shape of your own choosing, like a mecha, by maintaining the design language and features you've observed elsewhere.
Basically, you will never be able to draw a convincing mecha from imagination if you cannot draw a convincing tractor/excavator/industrial welding robot from observation first. The skills that go into the latter are necessary in the former - not to mention the usual stuff like perspective, line quality/hierarchy, shape language, big/medium/small, and other fundamentals that go into a good drawing.
Hope that helps! Go copy some robots!
