Some Random tips I picked up when reviewing some of the web pages. The basic choices are:

Wood:
Hardwood:
  - IPE and other tropical hardwoods have the longest life 30+ years and greatest environmental impact and highest cost.
  - Black Locust and Sassafras: long lifespan (20-30 yrs), Moderate Cost, Can be oiled annually to keep color rich.
Soft Wood:
  - California redwood Western Red Cedar, Cypress:lowest lifespan (10-20 yrs), highest maintenance .
  - Preasure treated pine - Not recommended

Composite:
  - Trex - 25 years - Most expensive - No maintenance,
  - PVC TimberTech AZEK , fiberon - 50 years - No Maintenance - only 1" boards

See fiberon vs trex

The deck should be slightly lower than the floor insidce to prevent water damage from splashback from rainfall.

Wood decking requires a gap between the substructure and the ground. Having at least 7 inches of ground clearance is recommended to provide adequate airflow to prevent moisture accumulation, damaging the substructure and fixtures.

You may want to change deck heights either between spaces that join the deck or between deck levels themselves. For example, you may want to use steps immediately outside a back door to drop down to a lower level deck. Consider the configuration and size of your stairs to determine how many steps youll need to have installed to achieve the preferred height. Safety should always be a consideration. Steps can be dangerous and you should consider whether or not stairs right outside your threshold pose a problem. You may also want to consider a handrail if you are descending more than a couple of steps.

A well-constructed deck must have space between boards to allow the wood to drain, dry, expand, and contract without suffering harm. You must also leave the appropriate size gap because too much gap becomes a safety problem.

Deck flashing is critical when it comes to making sure you keep unwanted moisture (in other words, all moisture) away from your home and its foundation. Flashing connects your deck to your home while redirecting water away from it.

For a small space that fits a table for four to six people to sit around, about 150-200 square feet of deck boards is required. This size allows space for guests to relax and move around but wont accommodate much more than a simple sit-down dinner for a few people.
The average deck size is between 300 and 400 square feet. This size provides enough space for around 10 people to have a sit-down meal.

Bpard spacing: For kiln-dried wood with a relatively low moisture content of around 10 to 12% and a relative humidity level of about 35 to 45%, the final deck gap should be about 1/4 inch. However, if the deck installation occurs during drier weather, with lower percentages of moisture and humidity, you may need to add an additional 1/16 of an inch, to account for additional expansion.

Air-dried decking can often be installed with a gap less than 1/4 inch (6.5 mm) because the boards will likely end up shrinking in width rather than expanding. When installing new air-dried decking the proper gap ranges from 1/8 inch (3.5 mm) 1/4 inch (6.5mm)

Links:
How to Build a Deck in 10 Steps
WHAT SIZE DECK SHOULD YOU BUILD?
Renovating your backyard? How to decide what size deck is right for you.