For uploading the products on jet.com
Click Product -> Manage Products to see all the product details
Select the product whose status is Not Uploaded.
Simple products
Simple products can be upload directly.
For variants products(like color,size etc):
If you have variants product then you just need to map the shopify option with Jet attribute.
Please once go through the below shared Doc(how-to-sell-on-jet-com)
https://shopify.cedcommerce.com/jet/how-to-sell-on-jet-com#sec4-2