Don't bother preparing the soil too much ahead of when you want to plant... it will just sprout a bunch of weeds and you will have to do it all over. But momo is right. Right now is not a great time to plant most things, because of being pretty hot most places. Plants are vulnerable when first getting established and the height of summer is not a good time for vulnerable plants (though it can be done if you have time to give them very good care). But fall is a good time to plant lots of bulbs and perennials, shrubs and trees.
So sketch out the areas you want to work on and start making a plan. Think about what the effect is you are going for, how much maintenance you will have time and energy for, what you want to use it for (do you want to grow veggies, herbs, fruits, flowers, do you want to attract birds/ butterflies, do you want fragrance, etc), color scheme. Think about the views from your windows, deck, poolside. Think about where you will want paths and sitting area, water feature. Start by putting in the hardscaping (paths, retaining walls, arbors, anything you will build) first.
Once you've answered some of the above questions let us know if you want plant suggestions. I have a lot less space than you, but I have a deck, a patio, a little bit of veggies, a bird and butterfly garden, a patch that is asparagus/strawberries/ rhubarb/raspberries, an herb garden, a moon garden, and a native woodland shade plants garden. So dream big, you have plenty of room for it!