Bitcoin is a digital currency and you earn it the same way you earn your local currency; by offering a service and accepting payment in Bitcoin. This forum offers some means of selling such a service (promotion) through signature campaignsm. Also, if you have any skill you can monetize, start a thread in the Services board or browse through the board for any offers that fit your criteria and apply.

You should do the learning part first and after you've gained more knowledge about Bitcoin and the different facets to it like mining, you can then decide if it's profitable to get into