How to develop Blockchain Project?

Ethereum is just one platform for developing p2p blockchain networks, there are a lot of them and no every cryptocurrency is not a blockchain platform like Ethereum as someone stated, Bitcoin, Ethereum Lisk, Neo, etc. are all platforms you can develop on, many of the other projects you see on coinmarketcap.com are developed on top of those platforms.

Sure you can use blockchain for a lot, the real question is if you need it? and if it's suitable for what you want?
There are some use cases where implementing blockchain just doesn't make sense and then there are other use cases which the technology isn't ready for yet meaning you would have to use some middleware and in some way centralise yourself again.
So first figure out what you want to do and then see if blockchain would enhance its solution.