Function to return a multiple colmn table or view

I am new to psql , so please be patient !

Can someone please provide a small quick example of a a function that
take 1 paramater and based on that parameter, returns a table or view ?

Etc .... Note this is just a abstract of the functon, not a working
function call !!!

Create function viewtest( start_month)

If $1 = 'April'
Then
Select * from april_view;
Return ;

Let me know how to do this.

Thanks