You should use pressure treated wood to uphold through the elements,with the wood began building the deck frame for the landing.Your landing should be 5ft*5ft so that a person with a weelchair could turn 360 degrees without getting stuck.
Take a framing nailer to assemble all pieces of wood creating the frame(cut and attach cross joists to the inside of the frame every 16in)
When finished place the frame along the edge of the doorway and flush on the concrete.Make sure the frame is level(vertically & horizontally). Add shims(pressure treated wood)where it is needed to level it up and to provide ventilation.
Then attach the frame to the building(pushing the nail gun between the joists)apply nails every 8inches apart(insuring the deck is secure)
In order to create the ramp make sure you measure the distance from the top of the landing to the concrete base,cut three boards next when using these measurements on one end and angling the cut down to a point;this will create ramp supports.Next fasten all three ramp supports to the side of the landing
(use wood cross braces between each of the 3 ramp supports for more stability)Secure them to concrete.Then drill 2 pilot holes in each brace using a masonry bit & fasten using screws,then fasten all braces to ramp supports.
(next step)Add deck plaks across ramp & landing frame.Place a full board where the angle of ramp changes;orient boards making the crosswaysto the ramp.Secure each board,then when planks are in place trim off any overhang with circular saw making it even with frame and sand to smooth.