  Through the Schwarz lemma, we provide a new point of view on three well-known
results of the geometry of hyperbolic surfaces. The first result deal with the
length of closed geodesics on hyperbolic surfaces with boundary (Thurston,
Parlier, Papadopoulos-Th\'eret). The two others give sharp lower bounds on two
metric invariants: the length of the shortest non simple closed geodesic, and
the radius of the biggest embedded hyperbolic disk (Yamada). We also discuss a
question of Papadopoulos and Th\'eret about the length of arcs on surfaces with
boundary. In a sequel, we use a generalization of the Schwarz lemma due to Yau
to study the injectivity radius of surfaces with bounded curvature.
