How do you beat the king

Well I got ColII and turned out to be a fun games , but do you destroy the royal expeditionary force of 80 if you only got 18?

Well... get more units...
Just 18 Colonists (and just 80 REF units ) sounds awfully few - like in declaring indipendance before beeing prepared.

There are a few threads here detailing the mechanics behind REF growth and hence strategies to keep it low, while having enought population yourself, to beat it.

The REF will also land in waves, so you don't really need to match in man by man - just need enought to take on one wave.
And use terrain/promotion/unit bonus to your advantage.

can you give a link , please

http://forums.civfanatics.com/showthread.php?t=295464
http://forums.civfanatics.com/showthread.php?t=294767

There are two threads with collected Gameplay tips.

There are a lot more / more specific threads focusing on certain aspects. Just look through the Civ4Cols Strategy and Tips section and look what suits you.

thank you for the links

Simple answer is that you need a period to make LB push and then DoI. Before you need tome to get enough Statesmen, Horses, Guns to create troops after DoI.