The best way to begin is to understand that one kilogram is equal to 2.2046 pounds. From there you can easily do a conversion equation. You begin by multiplying the 140 kilograms by the equivalent of one kg in pounds (140 Kg x 2.2046 lbs), which will give you the answer: 308.644 pounds.
Practicing these simple conversions can come in handy. It's unlikely that you'll have to specify your conversion to the thousandths place (unless you are in a science class), so an easy way to perform the conversion when doing similar equations is to remember that 2.2 pounds are roughly equal to 1 kilogram.
For more information on conversions, visit a website with a conversion tool, such as metric-conversions.org. If you're still having trouble with conversions, you can always ask a teacher or professor to go over the theory of conversions with you.
Another good resource chart is a conversions table; this is a device used in many different professions that gives a breakdown of numbers and their equivalents in various units of measurement. You can find conversion tables in textbooks, resource guides and on the Internet, and you can even access tables from a smartphone via the web or conversion apps.