There are lots of ways to earn bitcoin. Let's enumerate them and then discuss.
Forum
Mining
Buy and Sell
Free bitcoin
Invest / Save
For the Forum we have two types which are the Signature Campaign and Bounty Campaign. Both of them you can earn bitcoin. Just have the said rank by posting. When you are a member or have joined already, post again base on their protocol.
Next is Mining, it is one of the way where you will just let your computer system mine the bitcoin for you. The rate of your money depends on how fast your computer system is.
Next is Buy and Sell, you can buy bitcoin and then wait the price to rise and then sell it to the buyers. By this method you can earn well. In this, trading system is also included.
Next is free bitcoin, some sites (some are legit, some are not) gives free bitcoin through roulette, raffle, playing games or rolling a die.
Lastly, you can invest or save. This is one of the good strategy so that you can earn a good amount of bitcoin.