Get a smaller shed and burn that down, I bet there's a bunch of junk you never use in that thing anyway. Put the new shed back closer, then remove that useless floor and fake grass. The soil is probably ruined with construction sand, dig out a palm of it and throw it away, get new soil. Ask the gardening store which soil they got is best for cultivating vegetables and what best fertilizer they got, make them give you instructions on how to manage that shit properly, write them down if necessary.
Get that shit and fill the hole, then follow the instructions and try planting onions to see if it's working properly, they grow fast once you plant them, try both white and red onions so you can see if they react any different. After 2 weeks show them to the store owners and ask them it they're growing alright, it's easy to misuse fertilizer and ruin the soil again, you've to be careful.
Once the soil is confirmed ready, plant whatever you want, just keep in mind distancing and light exposure, when you buy vegetables from the garden store they almost always come with instructions, if they don't have instructions ask the store owners how to treat them properly, always take notes if necessary.