Bitcoin has now become the one of the leading currency.It's not easy to earn bitcoin. Either you have spend some money from wallet or you have to spend time understanding Bitcoin.

So some information to earn bitcoin in my opinion:
 
*Buy and Hold for long term
*Research and invest in project

I think this is the best way to earn bitcoin because many scams and fraud is happening in the filed of cryptocurrency.
If you don't want to invest own money you can join airdrop,bounty,etc