Hooray for your new garden!
I think you sound like you already know what you're doing - you're going to be soooooo fine!
Grab a good book and have a sit in your garden for a whole day (preferably when it's lovely out). Watch where the sun goes. Make a plan of where it's shady and where it's sunny every couple of hours, so you don't end up siting a patio or veg garden in the dankest, wettest corner of the garden or a lawn in deep shade. Accepting what you have and working with it is so much easier than trying to do something that runs directly counter to what nature wants to do in your plot. When you buy plants, don't be seduced by the lovely-looking specimens in the garden centre: check whether they'll actually grow in your plot first! And also whether they are hardy, because the last thing you need is an entire border that needs to be removed and stored all winter because it's tender. (Doing this with a couple of plants is OK, though!)
Make a list of all the things you want in your garden - play area, sandpit, pond, lawn area, herbacious borders, wildlife area, veg area - whatever. Then work out the practicalities - if you work from home and have small children who like playing outside, it might be a good idea to have a lawn in sight of the house, for instance. Try to figure out how to join the things you want together in a way that provides not just a flat 'plan' but a journey, so you can walk through the garden and be delighted and surprised - use big plants or features to screen areas off so that you can turn a 'corner' and be in a new area. This is possible even in very small spaces, with some thought.
Fill loads of pinterest boards with all your ideas and anything that inspires you. Such a great resource, and free too. Have a look in neighbours' gardens and see what grows with abandon, because chances are it'll work for you too.
Once you've got a rough idea in place (and don't worry, things will inevitably change as you go on), the main thing is not to shy away from the work that needs to be done at the start: you've already grasped that (proverbial and literal) nettle, which is great. You know the soil contains loads of rubble - so that needs to come out wherever you want to plant, to about two spades' depth. Buy a mattock, some rigger gloves and some of those strong plastic trugs and give up your gym membership because you're not going to need it for a bit! Then add loads (and I mean loads, think: measured in bulk bags) of compost, perhaps with a bit of manure if you think the soil is poor (upgrade that to lots of manure for veg). Add gypsum and grit for heavy clay.
Then plant, and enjoy your new hobby! Welcome to a world of obsession!! 