How can I repair this rotted section of wood siding?
Frankly, replacing one horizontal run like that probably isn't worth the effort, unless there will be lots of rain in the future.  It may well be sufficient to strip, putty the gaps, caulk the edges, and paint.

Otherwise, you'll have to replace the siding panel.  Remove the bench and wall.  Remove the siding, and renew.

The tools and materials are straightforward:  hammer, pry bar, screwdriver (if anything is held with screws&mdash;probably nothing is), elbow grease, and patience.  Nails or screws, a new siding panel, primer, caulk, and paint is all that should be needed.