I'm new to blender and I don't even seem to be able to extrude my faces straight down. I'm creating the base mesh for a shirt, but I keep extruding faces at different angles. Some help would be appreciated!
Immediately after typing E (extrude), type Z twice: first to override the default direction (along the faces' normal), then to constrain the extrusion along the Z axis.
(E, Z, Z, move, Enter. Note "along global Z" on the bottom bar)
To add on this: you can constrain to an axis - X, Y or Z - also the other familiar operations G (move), R (rotate) and S (scale).
Similarly, Shift+Z constrains the transformation to all axes except Z.
Finally, selecting an axis twice means that the axis local to the object's internal coordinates is used, rather than the global one.