Howto format a float8 number?

Hello,

I'm sorry, I look all the documentation, but I didn't find how to format
a float8 number.

I want to print a floating number like this :
    10.1 -> 10.10
or : 10000.1 -> 10'000.10
or : 0 -> 0.00
etc...

If you have some idea, write my!

Thank a lot.

JK