How do you use the metric system?

You use if for weighing, measuring length, measuring volume, and many other things. If you have a specific question, post it.

How do you use it? for instance... 1000mg=(blank)grams and how do you figure it out?

You memorize the metric prefixes.

m = milli = 1/1000
1000mg= 1000 * 1/1000 grams = 1 gram