There are so many ways where you can invest on bitcoins. Imagine this is an equivalent to money in the real world. So what do you do with money to earn or gain from it is the same as using bitcoin. The simplest one is to buy bitcoin from exchanges and then sell it when you have gained. Other things you can do would include selling things online and get paid on bitcoin, or maybe create an exchange of your own, create your own lending company with bitcoin as currency, getting paid for services in bitcoin and many other things.