Click on Channels -> Add New Channel menu at the left menu bar.
Select the type of channel you want to connect from the left tabs as shown below.
Enter the required information for the chosen channel. Some of the channel types may ask you to complete authorization to connect.
Follow the below links to see the detailed instructions of connecting different type of channels in details.
How to connect Amazon Seller Central account.
How to connect eBay seller account.
How to connect Walmart seller account.
How to connect Etsy seller account.
How to connect Jet seller account.
How to connect Bigcommerce store.