Does anyone know anything about gardening?

Ok long post warning.

I have a garden. I know nothing about gardening. What I do know is that I want to do something with the bland rectangle. I’ll break this down into several questions:

1. The ‘grass’ is mainly moss, cat shit and builder’s rubble. What’s the best way to resurrect this? Dig the whole thing over? Pick out the shit and rubble and scarify it?
   
2. I plan to make some (fairly low) raised beds along the left side using railway sleepers. The garden is on a slope so I want to even this out as much as possible and step the sleepers. What should I use against the fence to hold the soil and prevent it rotting? I’ve assumed the left side is the best side as it gets sun most of the day and the other side is almost permanently in the shade. Is this the best approach?
   
3. Does anyone have a good source for sleepers?
   
4. We plan to build a studio at the bottom at some point in the future (hello Golf Club thread). I’m guessing it’d make sense to run power under the raised beds while I’m digging in that area?