The tutorials listed here show how to create or modify geometry.
Points
Points 101 An introduction to the Point3d and Point objects
Points 201 Create an associative point feature
Lines
Lines 101 Creating unassociative line objects
Lines 201 Creating associative line features
Arcs
Finding arc start and end points The line object gives you direct access to the start and end points; the arc object does not. Here are a few methods to find the arc start and end points.
Arcs 101 Creating unassociative arcs