To start with, it depends if you want to build your own blockchain from scratch or build on an existing blockchain platform. If you plan to build from scratch you need to know the basic structure of a blockchain, youtube can help you with that, also you need to know some programming languages like C++, Go or Java. a blockchain as we all know is a distributed ledger. To build on existing platform I suggest you look into ethereum smart contract or waves platform.
I want to build on existing platform. Thanks for your suggest. I will try it