format a string in decimal (0,00)

Hi all,
I'm a beginner with asp net 2.0
i have a text box and i enter a number in this textbox and i would like to obtain as a result the number follow by ,00
exemple :
if a type 15 => ouput => 15,00
if a type 35,1 => output => 35,10
and so on !
how i can do this ?
i've try to use convert.ToDecimal(string in) but that doesn't work !
Thanks for your time
christ 
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