There are a few ways to go about this step, and picking the right method will depend on where you're getting your products from.
B) If you're looking to drop-ship products with your own designs on them (ie. tshirts, hoodies, bags, shoes, totes, etc.) then I would recommend using the Printful app.
C) If you have already found a supplier, and they have provided you with a CSV spreadsheet containing all of the product details, then the easiest way to import the products into your Shopify store would be through the Syncee app, as it will convert the CSV file into the correct format to import in Shopify (saving you potentially countless hours of work). This app also has an auto-updating feature, for if the CSV file is hosted online and updated by the supplier.
D) Alternatively, if you have a CSV spreadsheet file, and want to upload it manually (without an app) then you will just need to make sure that it matches the correct Shopify formatting (see here) before uploading. The formatting does have to match perfectly - with no errors - in order to upload successfully, which can take some time, but is usually doable with some simple copy-and-pasting and rewording of column headers.
E) If you are selling products that you manage the inventory for, or if you're selling services, then you will most likely want to create the products manually, one at a time. To do this, follow the steps below.
Go to your Shopify admin > Products > Add new product.
Put in the product's title, description, and add at least one image.
Put in a product vendor & type (note: these are just used for organizational purposes on the back end -- customers don't need to see these labels). Also add any applicable product tags (these are not the same as keywords). Product tags are useful if you're looking to auto-generate which products go in specific collections.
Add the product's pricing, inventory, and shipping details.
If your product has variants, you'll need to check the box for variants, which will then open up another set of options.
Once you've saved your product, the variants grid will show some extra options for editing, clicking the option to assign an image to each variant. This is important for if you want the main product image to switch images when a customer has chosen their product options.
Select a variant by checking the box next to it, then click "More Actions" and then "Add Images" ... then finally, choose the image (or upload a new one) for the given variant. Repeat this for each of your variants (if multiple variants have the same image, you can bulk select them then 'add images').
Verify that all of the information you've put in is correct, and save any changes. Repeat this process for each product you need to add.
Need more help with this step? Check out Shopify's help docs for adding products by clicking here.