Hi Mohammed and welcome to the forum,
In the case of setting up a Drop-shipping site of your own, then once you have your site built. You then need to get your products, so that's a matter of researching suppliers and opening accounts with the one or ones that you feel provide the best fit for your business.
For example, if you offer worldwide shipping, then you need to ensure that the suppliers you work with offer the same. So ensure your policies match what your supplier's policies are or you'll end up in a serious mess with your customers.
The way a Drop-shipping account works is that the supplier will provide you with your buy price. It is then up to you to set your own sale price on your website. Now it's not just as simple as adding some crazy percentage to that of your supplier's price and expecting to make sales.
You'll need to research the products you are selling to ensure you are competitively priced and that your service is also competitive with other suppliers.
Once a sale is made on your site, you will then need to quickly process the sale through your account on your supplier's site. Now one problem you may face is that your customer's payment may not clear into your account quickly, especially if we are talking about a new PayPal account. So you may be in a position where you will need to sue your own money to get the order processed quickly and sent to your customer.
When you are processing the order with your supplier, you give them the delivery address of your customer and they ship directly to them. The margin you make on the sale is the difference between what your supplier is selling the product to you for and what you sold it for to your customer.
Just give me a shout out any time you need some clarification on any part of the process.