Sketchup can be used along with ModelConverterX to compile it into an FS BGL file. For that you will need the file bglcomp.exe from the relevant SDK, along with its helper files (found in the same folder).
You may need:
Sketchup to create the 3D objects. It can also create ground polygons (for custom runways, taxiways, etc.).
ModelConverterX (MCX) to compile the Sketchup objects and ground polygons.
Airport Design Editor (ADE) to create or edit the airports (default runways, taxiways, etc.). It can also create ground polygons (for custom markings, etc.). ADE can also create simple terrain and exclude polygons (to make the harbor water/land shapes, airport background polygons, and shorelines, roads, etc.). It can also place your Sketchup objects, although not directly within the sim.
SBuilderX, if needed to create more complex terrain polygons and lines (not compatible with P3D?).
The relevant SDK.
A paint program that can create (at minimum) 24 bit BMP files.
DXTBmp, Imagetool or Photoshop to create the final DXT BMP/DDS texture files.
Optional - a program like Instant Scenery to place the objects directly into the sim, making it easier than using ADE.
There's probably more, but this will get you started. There are also some alternative programs to those I've mentioned above; I'll let others point those out.