Newbie: How to record basic company expenses

I’m new to Dolibarr and want to find a simple way to enter a company expense when buying materials/supplies from an online store. I assume you add the online marketplace as a Vendor, then create a Vendor Invoice, but from there I’m basically supposed to create and enter products with prices? This gets tricky when it comes to sales tax. When I buy something online, how do I copy the line items minus sales tax but then add sales tax at the end? Can someone point me to some resources on this? The video tutorial is pretty basic and doesn’t go into taxes from what I can tell.