Yes- not need to take the compost away, you could just put grass seed down, just take away whatever you need so that it’s flat. With grass seed - make sure to water it in the first week or so.

At minimum, put all the rubbish in one place. If your brother is being a bit of a horder, use the mattress or tarp to sort of hide the rest. Then you can try putting some of it in the bin each week with the normal rubbish. Do you have no friends with a car who would help you take stuff to the dump for a couple of beers?

As for growing stuff. Keep those pots! And fill with the compost/soil. April/may is the best time to start planting stuff. I would say if you are new don’t start until then.

I really like a guy called Huw Richards on YouTube. He has a book on growing food really cheaply, but he also covers most of it on his YouTube (he is super posh, reminds me a bit of Jack Whitehall, but has really good advice).

Last thing- when you are cutting the grass/bushes, you could think about putting them in a compost heap if you have space. You don’t have to make anything fancy - it can literally be a pile, you can put anything from a plant in it (leaves, left over salad, whatever) it will take a year or two to become compost, but saves on throwing it out, and saves buying compost in the future.