How I create my function in sql server 2008?

I want to create my function that can retrieve data from Table in sql server 2008 on the basis of several parameters which I pass to that function as argument?

And I also want to make that function in such a way that if parameters contains a value except null then this parameter should add to select query in such a way that

select * from Table where

If(param1.value!=null)

param1=value of param1

.....

and it is upto the all parameters which I send in function as arguments and in last I want a select qry which would be like

select * from Table where Param1=Param1.value and Param3=Param3.value and Param5=Param5.value

in above qry Param2 and Param4 having NULL values.