start a career in blockchain technology?

As is the case with many fields, the field of Blockchain Technology is very vast and there lot of opportunities in the field.

Career as a developer:

Understand the basics of Bitcoin Blockchain - read the whitepaper, watch some Youtube videos etc.

Understand Ethereum - read the whitepaper, watch Vitalik Buterin’s videos on Youtube etc.
Understand various types of Blockchains and their specific use-cases: public, private and consortium.

Learn about smart contracts and start coding by reading docs, articles etc.

I would recommend to start of with Ethereum Blockchain development (learn solidity) and then move on to Hyperledger, Tendermint etc. depending on the specific use-case you are developing for.

A developer should be active on online communities on Reddit, Gitter, Stackexchange etc. as the Blockchain industry is quite nascent and changing rapidly.

Career as a consultant:

Many companies would be planning to adopt Blockchain technology with time.
They would need people who understand it’s use-case and how it’s going to pan out in the future. People with a good business sense would be ideal for this kind of role.
I recommend these type of people to understand the basics of Blockchain Technology by reading the bitcoin whitepaper and then go on understanding the use-cases of Blockchain in various fields like supply chain, identity management, IOT etc.

List of companies for the same:

1)IOT + Blockchain - Chronicled

2)Supply Chain (specifically diamond) + Blockchain - EverLedger

3)Company At The Forefront of Blockchain - ConsenSys

4)Leaderboard - The 100 Most Influential Blockchain Companies - Week 44