How can I use bitcoins at bitstamp to buy something from some online seller?
If you want to buy something with bitcoin, you either need to find a seller who accepts bitcoin as payment, or you need to convert your bitcoin into a currency that seller will accept.
Assuming that the seller in question accepts bitcoin, you will need to send that seller bitcoin in whatever way the seller has set up for you to do so. Usually this means clicking a link, scanning a QR code, or copying a Bitcoin address into your wallet software. From there, the seller will see your transaction in the mempool, probably wait for at least a single confirmation, and then process your order.
If the seller does not accept bitcoin, you will need to convert your bitcoin into a currency that is accepted. This can be done at an exchange (such as bitstamp), using whatever process and rules that exchange has in place. Usually, this means tying your account with them to a traditional bank account, so that the exchange can deposit your fiat currency into that bank account. From there, simply pay the seller in whatever means your bank provides.