Think about how you will use your landscape. Will you entertain and BBQ, do you need a childrens play area, are you a gardener? Ask yourself what you want to see in your yard and include things like fences, decks, patios, fire pits arbors, seating & grass areas, sprinkler system, plants & trees.
Set a budget. Allow about 10% - 20% of your home value. If this seems like a lot, remember that it includes fences, decking, grading and landscape. Also consider that appropriate landscape improvements are estimated to return 100 - 200% of their cost when a home is sold.
Look at books and magazines for ideas to show your landscaper. Keep a file of what you like and what you don't like. It will help you achieve your goal.
Think about how much time each week you can allot to maintenance needs.