I am using SQL Server 2008 and I need to run a SQL Job from SQL Server Agent. I am new to SQL Server Job and I want to execute a stored procedure regularly from a SQL Server
You just need to add this line to the window there:
exec (your stored proc name) (and possibly add parameters)
What is your stored proc called, and what parameters does it expect?
As Marc says, you run it exactly like you would from the command line. See Creating SQL Server Agent Jobs on MSDN.