You will find a list of all applications in here. They are organized into different categories to help you find the right integration faster.
Select the appropriate add-on and click on 'Install add-on'. You will also see its price. Most integrations are free. It is also worth to pay attention to the author - if you have questions, you should contact them.
Then, enter your store name, click on 'go to your account', and on the next window confirm the installation.