I would recommend you to focus on web for now as mobile is still on its early days.
If you want to handle ether you have to build a smart contract. These are written in solidity and you can see a very first example of how to create a contract where you can send and withdraw ether in this chapter of the Ethereum Book. I strongly recommend you to read the whole book, it's free :).
If then you have the basic idea of how to build and deploy the contract, you can start thinking about how to add "data" to the transaction. That data will be the message you want to hash. Read more about it here. What this chapter explains is the structure of a transaction, where you can send value(ether) or data (your message).
Then you will have to start thinking about how to display the data of that transaction into the UI of the page you built. The way you connect the smart contract to the UI (html, css, reactjs..) is via a library called Web3js.
There are many things to consider here, if you are just starting its gonna take some time to understand the parts but I encourage you to start!
Some of your friends will be: Ethereum Book, Remix, solidity, and the whole community :)
Hope it helps, if you want to ask something else I will try to help