Understand the reason for the metric system. The system used in the United States is based on an English system of weights and measures in use at the time of the founding of the American colonies in the seventeenth century. During the next two centuries, the European scientific community developed a system of measurement based on agreed-upon standards, using the same number system as mathematics--the decimal system--to develop larger and smaller measures. The United States adopted the decimal system for its currency but the old English system of inches, feet and rods remains in use.
Learn the basic units. Meters measures area: liters (l) measures liquids, grams (g) measure weights and so on. Unlike the older, local standards, which might be based on such arbitrary measures as the length of the king's foot or distance between his elbow and wrist, these units were created using measures of inert, unchangeable materials. There are measures to quantify atomic weights (moles), the wavelength of light (Angstroms) and luminous intensity (candela); of temperature (Kelvin units) and electricity (amperes).
Learn the divisions. Metric units are expanded and divided using multiplications or divisions of 10. Latin suffixes are used. "Milli" is one-thousandth (as in ml), "centi" is one-hundredth (as in cm) and "deci" is one-tenth. "Deca" is 10 of a unit, "hecta" is 100 and "kilo" is 1,000 units. Kilograms, in common usage, qualify as a unit since they are used to measure so many things, a kilogram being roughly equivalent to a bit over 2 lbs. Divisions that have been added in the last 50 years include "mega," "giga," and "tera" (million, billion and trillion) and "micro," "nano," and "pico" (millionth, billionth and trillionth), adopted in 1961 at the dawn of the computer age.
Learn the way to measure. Since each unit can be divided into 10, 100 or more (by a power of 10) unit, decimals can be used to convert and communicate units easily. For example, 3,482 g becomes 3.842 kg. No need to remember how many ounces make a pound (16) or pounds make a ton (2,000). Once you begin to think in 10's, conversion becomes a simple matter of moving a decimal point a certain number of spaces.