when using paypal payments standard, the checkout form must post the information directly to paypals servers.
you cannot have the checkout form post to your page to store the information, then pass it along to paypal.
PayPal standard uses the following flow:
checkout page: User enters billing and shipping info, this posts to the confirm page.
confirm page: order summary and order details are stored in database. Order summary table should have an "Order Status" column, set to pending. the information form the checkout page is stored in hidden form elements, this posts to the paypal server to process the transaction.
Here's where the difficulty of Payments standard comes in
you will need to configure your paypal account to use IPN (Instant Payment Notification)
you will set paypal to post to a page on your site when the transaction is finished processing. This page will be configured to update the order table to change the status column based on the transaction state (Success or Failure).
PayPal Express checkout is a much simpler to use free alternative to Payments Standard.
I would recommend using the eCart checkout wizard to create the checkout pages for you and use Express checkout instead of Payments standard.