Very straight forward.
Firstly you have to choose the supplier/s that you want to work with based on your selection criteria. Once you do that, simply open an account with them and then you are able to list their products for sale without paying for them upfront. You only pay for them when you sell something, then you process the order and they ship direct to your customer.
You make your money from the difference of what your selling price was and what the supplier's cost price is. Just bare in mind that you need to calculate all your costs when pricing an item. Listing fees, selling fees, transaction fees, shipping fees, absolutely everything or you can quite easily end up loosing money on the deal.
I notice that you use the term wholesaler in the same post as dropshipper. Just be aware that they are two completely different things. A wholesaler sells products in bulk with a set minimum order quantity, the more you buy, the cheaper the price. A dropshipper has a set price per unit, whether or not you buy 1 or 20 of the same item, they have no discount on bulk orders, but have no minimum order quantity like a wholesaler.
Dropshippers like to call themselves wholesalers a lot of times because it's a good marketing term. People associate the word wholesale wholesale with cheap, so it gets thrown around a lot these day, but isn't relevant unless the supplier is offering you discounts on volume purchases.