My yard is too soft and sandy. How do I thicken it up?

We just moved into our new home and didn't realize how sandy our backyard was until we moved in. While moving heavy yard furniture into our new backyard with a pick-up truck, our truck got stuck for four hours until we finally gave up and called a tow truck to get it out. It wasn't well kept by the previous owner, but it does have some grass. I need to know how to thicken up the foundation so I am able garden and build a back porch without it sinking.

My yard is too soft and sandy. How do I thicken it up?
Back porches or decks are secured with a concrete footing. I usually dig down till I hit solid ground and then place the concrete in the hole before I install the deck support posts. The sandy conditions are probably where sand was dumped to mix with mortar for laying brick or block foundations. It is probably not very deep. Usually the contractor will bring in top soil to cover these areas and sow grass. The grass root system is what strengthens the yard. Time will allow the root system to take hold. If the conditions are beyond this, you would need to bring in more top soil and re-seed your yard.