Your initial thoughts
Start by assessing your future needs for your garden, who is going to use it and for what purpose. Does it need dividing up into tranquil areas for more mature family members or perhaps play areas for children or pets, do you want to attract wildlife like garden birds ? And what about maintenance, do you simply want areas of grass or do you have the time and skills to develop the classical country house borders of mixed shrubs and herbaceous plants?
Develop a theme
Thinking in terms of the overall look if your house is of a striking contemporary design you may find that architectural plants with bold foliage and areas of gravel and paving in finely dressed stone or concrete may be suitable, whereas if you live in a traditional country cottage fragrant honeysuckles and roses with winding paths in brick or roughly hewn paving slabs are preferable.
What to do next
Keep a notebook of your initial thoughts and perhaps do a rough sketch as well. Take a walk around your garden, taking a hard look at things that are past their best. Are paving slabs loose or broken, does the pond leak, are existing plants to your liking, over-mature or gappy? Are the existing features where you would like them to be, are there views that can beneficially be opened up or things that require hiding from view? These are the sort of questions that you need to ask yourself.
The next stage is to get out into your garden and mark out what you intend to do, some sticks and string are useful or the kind of marker paint that is used on building sites. This is available in a variety of colours, you can use a different one for paving, ponds, planting, or lawns etc. Take care to allow plenty of width for paths, space for seating on paved areas and lawns if required, and allow plenty of space for plants to grow and mature into. Take a few photos from different angles of what you have marked out for future reference.
Take time to reflect
Now refer back to the notes that you made earlier, is what you wish to do practical, does it fit the available space, can it be achieved and is it possible within your budget? Do any of the features need moving around from where you initially placed them, do others need to be added or even discarded? Again, these are the kind of questions that you need to be asking yourself. If at this stage your thoughts have turned into a bit of a fog you may benefit from some advice from a professional garden designer, otherwise you are now ready to proceed to with your project.