Mark where the sun goes at different times of day. It will change a bit but if an area is sunny now it will be sunnier come summer. Check for shade from trees that aren’t out yet.
Then make a list of how you want to use your garden. Eg evening bbq, seat in morning sun, wildlife friendly, kids play, compost/storage, lawn, flowers, veg, herbs.
Mark out each area then plan paths to join them.
Ask for cuttings/divided plants from neighbours. Have a look in their gardens to see what grows well in your climate. A good rule of thumb is 1/3 evergreen plants so you have some structure in winter.
Once you have a rough plan I’d start with one area first so you don’t bite off more than you can chew.
Try putting in some cheap climbers (honeysuckle,roses, clematis) next to the fence for a quick and easy backdrop, it makes a massive difference. You need to remove the grass 1’ around to stop it getting smothered.
Reuse materials, eg upside down glass bottles almost buried make a nice border edging. Or logs and sticks, old bricks etc.