How can I build a DeFi lending protocol?

The complexity of the protocol you are building depends on the kind of platform and the number of functionalities you want your platform to have. The simplest thing to do may be a trading or decentralized exchange, when you decide to add a borrowing and lending feature, along with an awesome user interface like those on Uniswap, you will be taking things to a whole new level which will require some extral work in front end and UX aspect of your project.

Since you are interested in a lending protocol, you can find all the help you need on YouTube, and since you are already a developer, you may find it easy to follow along with the tutorials. Note that you have to write code in Solidity, a programming language that has a similar syntax to Javascript. If you want to build on Solana, then you should know the Rust programming language which is a bit more complicated. There are other tools like Ganache, Truffle, and web three wallet test net which will help you take your project from something basic to a complete platform. You can find detailed information from EatTheBlocks or DApp University on YouTube.