You've just successfully set up your account and downloaded the Amazon Seller app, now how to source:
The very first thing to do in the app if you are planning to sell FBA, is to go into your settings (3 horizontal lines -> gear icon -> product search) "Default Fulfillment Channel" and switch it to "Amazon Fulfilled". This will make it so when you are scanning and analyzing products, the numbers and pricing you are quickly quoted will by default be including the Referral Fees and Fulfillment Fees. If you leave the app on the default "Seller Fulfilled", the fees will not include Fulfillment Fees and your profit will be a lot lower than you thought.
Once that is handled, return to the main screen and hit "Add a Product" from the menu. There are 3 main ways to look for products on the app, you can either:
Scan the items their barcode by clicking the camera icon
3D scan the item itself, like the label and branding, and see what matches come up or
Type in the item by name in the search bar
I use a variety of all three while I am sourcing.
Before diving in and going to a store, look in your pantry or cupboards and scan one item using all 3 methods and see the differences in listings they bring up. Next, scan as many items as necessary to get more comfortable with the app. Who knows, you might find a product you use and notice a large disparity in price and have your first item to go purchase for resell! (Do not sell the one in your pantry without a receipt.)
You'll notice that some items will show up as "gated", Amazon "gates" certain categories and brands to new sellers until more trust is established within your account. I explain all about gating and how to get ungated in detail here.
Beginner's Advice:
Try to work your way through a variety of categories, i.e. office supplies, sporting equipment, clothing, lawn and garden, etc.
Always remember to go wide and not deep on products and categories, there's a lot of trial and error in the beginning and it's best to learn with least risk.
Set strict rules for yourself on profit and rank requirements in the beginning, with time and experience you can bend or adjust them as you're ready to.