Where do you plan to purchase something?
If you're planning to buy a product or something from a store online that accept bitcoin when checking there is a new page will pop up.
This is what it looks like
For bitpay this is what it looks like
Just scan the QR code or copy the address under the payment checkout is the bitcoin address of the seller.
After 3 to 5 confirmation the checkout page will shows "This invoice has been paid" or just "Paid".
If you are dealing with someone here in the forum follow the AdolfinWolf's suggestion but don't forget to use escrow for safety deal because there are many scammers out there.