Building a patio deck is a little different than building a higher elevation deck. Because the deck is built at ground level it will not require a guard rail but offers its own set of challenges. The first obstacle to face is determining the amount of clearance you have available. Measure from the bottom of the door threshold to the ground at the house. If you do not have enough room for the framing you will probably have to excavate the yard. In any case you will probably not have enough room to install a girder or cantilever beam. You will most likely need to use flush beam construction. This method uses doubled up rim joists as beams set at the same elevation as the joists and attachments using joist hangers. You may need to bury your support posts because you don't have room for post base hardware above grade.
Because the deck will be in close proximity to the ground you should use pressure treated framing materials that are rated for ground contact. Also verify that your decking materials can be installed at this proximity to the ground. Some ground level patio decks may not require permits if they are free standing. Check with your building department for more information. Frost footings are recommended and may be required. A free floating deck set on the ground may be damaged as the ground experiences frost heaving.
If your yard slopes you will need to take measures to level your deck frame. Because most patio decks don't include guard rails they may seem empty or naked. Adding built in benches and planters can provide some definition for the deck and create a visual barrier between the deck and yard.