You've got to know all the basic stuff.
1 km = 1000 m
1 m = 100 cm
1 cm = 10 mm
and so on.
The tricky stuff comes when converting volumes to capacities. Probably best to memorise these:
1 m3 = 1 kL
1 cm3 = 1 mL
and work it out from there.
Also, there's these from the Spherical Geometry:
1 nm = 1.852 km
1 hour = 15
1 knot = 1 nm/h
and so on.