Deck plans can be obtained from most major lumberyards and some hardware stores. The deck that you have described is large enough, and consequently costly enough, that you should consider engaging a landscape architect to help you with the design. The money spent on design can pay huge dividends in terms of usability, aesthetics, and resale value. With respect to building around a tree, make sure that you provide enough clearance between the deck and the tree to allow the tree to move in high winds. If you are in an area that receives a considerable amount of snow, be sure to go under the deck and insulate the base of the tree with straw or similar material before winter. The snow, which normally acts as insulation, will not be able to get to the ground, after the deck is built. When installing a deck boards, be sure the boards are installed with the crown up. With the crown down, the boards will cup and collect water. Do not drive the nails or the screws too far into the deck. If you do, water will collect and the small holes and rot the deck more quickly than normal.