The correct notation is 5'8". This signifies you are 5 feet, 8 inches. 5.8 is not the same thing, because .8 is 80% of 10. This system is measured by incriments of 12. 8 inches is ~67% of 12 inches.
To convert, 1 inch is roughly 2.54 cm. So you take
5 feetx12 inches=60 inches+ 8 inches= 68 inches x 2.54 cm=
Your height in cm.