To enable the console, press "~" and type "sv_cheats 1" in it. Now, you can use one of the following codes to unlock the corresponding effect:
sv_gravity #: change gravity (replace # by any number)
r_shadowids 1: to move numbers on screen showing players' shadows
net_graph 0/1: (don't)show statistics at bottom of screen
cl_crosshairscale 4000: small crosshair
impulse 101: get 16,000 cash
phys_pushforce #: to make objects fly
no_clip: unlock flying around the map not getting hurt