As another pp says, measure it and draw it on a large-is sheet of paper. Also mark in where you have sun and shade at different times of year.
Make a list of what you have, and what you want to keep where it is. Draw the things you want to keep on the plan.
Then make another list of things you'd like to have. See where they would be likely to fit in to the existing plan, remembering things like a washing line, bins, bike storage etc. You could draw them to scale on bits of paper, and move them about on the plan till they fit nicely, like you were reorganising furniture.
Take photos out of all the windows that face the garden. Is there anything unattractive (like a bin store) or a view you want to disguise? That will give you an idea of where you could plant things to draw the eye away from the view, or for screening.
If you are deciding where to put things like flower beds, then you can mark them out with hosepipe or a trail of play sand. Then go in the house and look at it from all angles. That will help you see what looks best.
Start looking at things in neighbouring gardens to find out what grows well in your locality, and that will give you an idea of what to have in yours. Get talking to them, and you never know - you might get given all sorts of seeds, cuttings and surplus plants.