What do I do?

It,s not Taplow is it? Take some pics first.? The mound may have to stay becoming a feature? Books on terracing might be of help?

welcome to GoY. Have a look around at any gardens in similar situations. take photos too. is your property a new build or is it an older house and garden? If it is older what have the neighbours seen growing etc.
sorry not much help at the moment.

Don't panic. Rome wasn't built in a day. A little bit whenever you want. You'll be surprised how things start to look better every time you go out there.

Spot on Burgundy

Welcome to GOY!

My garden started off as a builder's rubble dump underneath the 'extremely thin' layer of topsoil! I found old railway sleepers, bricks, concrete blocks, lumps of concrete, you name it! But as Burgundy says Rome wasn't built in a day. So I picked a corner and started from there, doing a little bit at a time until I'd cleared all the debris. Then I hired a big rotovator and turned the whole patch over.

Try and get some photos put on as has been suggested, and then people will have a better idea of what you're up against!

Make some trips to the tip and get rid of all the rubbish, before winter sets in. Then try to decide where you want borders, seating areas etc ( see where the sun goes, and remember to have a 'sunset' seat if possible). Then, over winter, dig over the new borders ( or just start with one or two for now). Get the preparation work done before Spring as then you will have ground ready for all those lovely new plants!