If God wanted us to use the metric system, he would have given
us meters instead of feet.
 Burton Walder

SI is here
It's easy.
SI stands for "International System of Units" (in French)
but we Americans call it the *%#)+@^! Metric System. The French did most of
the work around the time we formed the Unites States. We might have helped
but we were too busy being independent.
There are seven base units of measurement. They define: length, mass,
time, electric current, temperature, luminous intensity, and amount of a
substance. Only three of these are different from our units and interest
us in our daily lives. They are length, mass, and temperature. We also
commonly encounter the derived units for area, volume, and speed.
SI is coming whether we like it or not and it's easy enough to get comfortable
with it. Here's a start for you.
Prefixes
First you'll need to know these  well, a few of them.
Greek prefixes make things big.


Latin prefixes make things small.

prefix  symbol  multiply by 
deka  D  10^{1} (ten) 
hecto  H  10^{2} (hundred) 
kilo  K  10^{3} (thousand) 
mega  M  10^{6} (million) 
giga  G  10^{9} (billion^{*}) 
tera  T  10^{12} (trillion^{*}) 
peta  P  10^{15} (quadrillion^{*}) 
exa  E  10^{18} (quintillion^{*}) 
zetta  Z  10^{21} (sextillion^{*}) 
yotta  Y  10^{24} (septillion^{*}) 


prefix  symbol  divide by 
deci  d  10^{1} (ten) 
centi  c  10^{2} (hundred) 
milli  m  10^{3} (thousand) 
micro  u  10^{6} (million) 
nano  n  10^{9} (billion^{*}) 
pico  p  10^{12} (trillion^{*}) 
femto  f  10^{15} (quadrillion^{*}) 
atto  a  10^{18} (quintillion^{*}) 
zepto  z  10^{21} (sextillion^{*}) 
yocto  y  10^{24} (septillion^{*}) 

* Prefixes, symbols, and powers of ten are universal but these words have different meanings in different countries.
Here's more.
Length
Start with the meter. Think of it as a slightly big yard. That's close
enough for most everyday use. Then there is the centimeter  think of it as
a half inch (only smaller). The kilometer is a short mile  well closer to
half a mile. Keeping these three things in mind will give you a good sense
of what people are talking about when they tell you how long things are.
easy to remember  more precisely

A meter is about a yard.  1 m = 39.37 inches

100 meters is about a football field.  the most common unit of meaure used in the United States

1000 feet is about 300 meters.  1000' = 304.8 m

About two and a half centimeters make an inch.  1" = 2.54 cm exactly

A millimeter is between 1/32 and 1/16 of an inch.  very close to 1/25"

A thousand millimeters is a meter.
Think of 500 mm as a half yard or 300 mm as one foot.  That's it: 1000 mm = 1 m

A kilometer is a really small mile.  1 Km = 0.62 miles

Speed
While riding in your car this is the same stuff we learned for length  a
kilometer is a short mile  well closer to
half a mile. So going a hundred Km/h isn't so fast (about 60 mph). Our
old 55 mph speed limit is about 88 Km/h.
You may hear "meters per second" occasionally. Multiply by 2 to get a rough
guess at miles per hour.
easy to remember  more precisely

You can go more kilometers per hour than mph.  1.61 Km/h = 1 mph

1 meter per second is two miles per hour.  1 m/s = 2.24 mph

Area
You probably won't encounter this often. Let's just say that square
meters are big square yards (almost 11 square feet).
easy to remember  more precisely

A square meter is a big square yard.  1 m² = 1.2 sq yd

Two and a half square kilometers is about one square mile.  2.59 Km² = 1 sq mi

Volume
You're used to this already. A liter is a quart  close enough.
easy to remember  more precisely

A liter is about the same as a quart.  1000 cm³ = 1 liter = 1 qt 1.8 fl oz

About 16 cc's make a cubic inch.  16.39 cm³ = 1 cu in

One of my elementary school teachers told me,
"A pint's a pound the world around."
(Please, tell me more.)
SI ties the volume of water with it's mass too  a liter of water is one
kilogram. Then they went a step further: a cube 0.1 meters on each side
has a volume of one liter. Make one with each side a tenth of that (or
one cm) and you have a thousandth of the volume (1 ml), or as you may have
noticed, 1 cc is the same as 1 ml and that much water is one gram.
Expressed in SI, 1 g of water occupies 1 ml or 1 cm³.
Mass
Think of a kilogram as a good solid handful  the weight of a 1 liter Coke.
The gram? Well, it's very little  about 30 grams in a one ounce letter.
easy to remember  more precisely

A kilogram is about 2 pounds.  1 Kg = 2.205 lb

500 grams or half a kilogram is about a pound.  454 g = 1 lb

A tonne (metric ton) is close to an American ton.  1 megagram = 2205 lb

30 grams is about an ounce.  28.3 g = 1 oz

Temperature
Celsius has 0 degrees at freezing
and 100 degrees where water boils  sounds simple but our 0 degrees
is not where water freezes so you can't convert by just multiplying or dividing.
But we can work with it easily if we remember a few things.
First, just memorize a few Celsius numbers:
 Ice melts at 0.
 Typical indoor temperature is 20.
 We got our number for human body temperature by converting from 37.
 Water is hot enough to burn you at about halfway from freezing to boiling or 50.
 Water boils at 100.
Along with this, keep in mind that Celsius degrees are about twice as big as
Fahrenheit degrees. This will be enough to amuse you the next time you
pass a bank.
For big numbers the 32 degree freezing point shift becomes insignificant
and you can just multiply or divide by 2. For example, if somebody tells
you the inside of the furnace is 2000 degrees Celsius, just think of it as
4000 degrees Fahrenheit (hotter than you can imagine in either case). The
same will do for large negative numbers like the temperature on Neptune.
easy to remember  more precisely

A degree Celsius is about 2 degrees Fahrenheit.  5°C = 9°F

One more thing: Kelvin is the same as Celsius except that
zero is 273.15° colder  at absolute zero  where everything stops
(almost).
Time
Well, time, measured the same way everyplace, right?  yes, but not
written the same way.
There's an international
standard but, guess what, we Americans don't use it.
A date is written like this:
YYYYMMDD
That's right, four digit year, dash, two digit month, dash, two digit day.
And time is similar:
HH:MM:SS
Yes, two digit hour, colon, two digit minute, colon, two digit second.
Of course the seconds are usually left off.
So, the correct way to write the current date and time is:
00000000
00:00:00
The best part is they naturally sort correctly.
Now you need to learn to speak SI.
If you would like to know more,
here is an interesting
place to start.