DAPP is a very broad term so it really depends upon the functionality you want. But in general it's going to require learning how to interface with a new network such as Bitcoin / Lightning / Ethereum. There are Lightning resources at https://dev.lightning.community/
For Ethereum you'd likely end up writing a smart contract so you'd need to learn Solidity - take care that it's easy to write a "working" solidity app that is insecure!