headers already sent

I'm doing a little form: In the first page i have the form to insert datas,then redirect the control to a script that check if the filds are correct.If they are correct redirect the output to a prospect else to the first page.The problem is that it gives an error like this "Warning: Cannot send session cache limiter - headers already sent " but I wrote "session_start()" in the highest part of all the scripts. I redirect the output with "header("Location: http://..") is it correct or there is an other way ? If i don't write "session_start()" in the html's page the variables dont appear in the fields(if i insert before,naturally). Someone help me please? Tanks Gian Michele