I used Shopify's drop shipping guide to get started--it really is very helpful. They're invested in making you profitable (it's in their best interest as well, since they make $$ from you), and consequently their guides are very good.
Fair warning though: you'll still have a ton of problems and spend time dealing with customers.
For example, the first time I got a $100+ sale on an item, I went to ship it and found that the vendor took the product down from AliExpress... but my Shopfiy account didn't update to reflect that.
So there are tons of pitfalls and things to be careful of, but like any field, you need to experience them firsthand, it's just part of the process. You can make money at drop-shipping, but it really is not easy, and there's still risk involved, like any business.
No, basically there are software add-ons to Shopify that let you import an AliExpress seller's product into your store.
Your customer never sees the original seller's listing, and therefore you can charge any price you want.
So, as long as you charge a price that is higher than the raw cost + S&H, you can then collect money from your sale, and use that to purchase the item from the AliExpress vendor (this is automated). Part of the automation process fills in your customer's mailing address, and the vendor fulfills their order directly. (So you don't have to hold any inventory, you're just the go-between.)
Your AliExpress vendors will know what's going on, but they expect and anticipate that people do this. It's how they make money.
Essentially, the service you provide is marketing, design, and salesmanship. They provide the product.
In particular, I'm wondering about how to handle problems - when orders don't arrive, when customers want to return defective items etc.