Im new to gardening, and i have a 6' x 10' concrete slab in my backyard from an old shed that has been since demolished. I would like to build a raised bed garden on this but have the below questions. any advice / tips would be greatly appreciated.

1. how about drainage? do i just put the soil right on top on the concrete? or do i put stone down first to allow drainage?

2. Do i have to worry about anything harmful leaching up into the soil from the concrete slab?

3. do i have to put a plastic membrane down on the concrete slab and on the wood walls of the bed ?

4. how deep of a bed should it be? (will be growing tomatoes, peppers, cucumbers etc)