it depends on what you want to do, what size of your ideas have.
Get a pen, a paper write your ideas down.
Then start with a design or your business model
I think the design (rough) should come out of your business model,
where your application design should come out of your rough design
In Rails, many Ideas may come also later while you are programming,
because you discover nice features in ruby and/or rails, which helps
you to integrate nice-to-have features.
But as I said, try starting with pen and paper write your ideas down
and make a sketch, of what you have in mind.
best regards and have fun!