If you have a good idea for an app, and the skills to create it, then producing your own premium app can be profitable. However, with so many free apps available, it may be hard to convince people to pay. If you want to go down the premium app route, consider having a basic free version to encourage people to download the app in the first place. You can then provide the option of upgrading to a premium plan once users realize how useful the app is.
To get started with Amazon affiliate marketing you will need to sign up with Amazon Affiliates. Then subtly market products on your site that are relevant to your niche and will be of interest to your audience. Make sure you only endorse high-quality products. If your visitors make a purchase on your recommendation but are not impressed with the item, you could quickly lose followers.