There are multiple options available for you to develop an app on steem platform. First of all, you have to first decide the purpose of your app. After you decide the purpose of your app, you can decide which technology you will be planning to use to develop the app.

If you are a javascript developer or a python developer, there is a highly sophisticated STEEM API available for you to use to build your app. The application that you build will utilize steem blockchain and will be decentralized. I'm not very great at Python but I will be able to give you directions for Javascript.

There is a Steem JS package available in npm which you can use to build applications. There is also a detailed documentation available on how you can use steemjs library. The repo link is available below. You can take a look at the documentation as well.

https://github.com/steemit/steem-js

There are also multiple tutorials available in steemit that can direct you and help you build your first app using Steem blockchain.