This will be fun. I am horribly inexperienced with side-scrollers

I will pick this reference to make sure that I know what I'm drawing:
http://www.kiddermathews.com/img/services/industrial.jpgand that's good practice. Unless you have things firmly into memory, you're gonna learn new things about what you're drawing if you simply use a reference
I look at proportional relationships between the horizontal and vertical supports, and I 'carve up' the game screen like this. I have a background, a tileset layer, and a layer for sprites (characters and goodies, what not). This helps me decide how to use my space on the game screen

And so, I decide that vertical supports will be 1-tile wide, and horizontal support sections will be 2 tiles tall, 6 tiles wide, plus another tile on top for the actual platform you stand on
Once I got size relationships down, I can then add as much detail as I want until I'm happy with it. Since this is just showing you a workflow I think is solid, this is something I wouldn't put in a portfolio yet, but this should be good enough what what I need

If you create tiles without having a feel for how they fit together in a level, something's gonna feel missing. It gets better with experience though, when you have a better feel for how you use your space for a more powerful game feel