There are two ways that I know where you can get bitcoin, one is through joining campaigns here bitcointalk forum and two is by buying bitcoins this is done through your e wallet which you can convert your cash to bitcoins. There are also other ways you can get bitcoin just like mining, trading and many more but I haven't tried it yet but will soon try it also when I become more knowledgeable about this things.
As you said you can get bitcoins as a reward from joining the signature campaigns but it is not enough for huge profits in future.Either you can convert your fiat into bitcoins by using the exchanges or you can mine your own bitcoins.Mining bitcoins needs investments for buying the miners.
Another option is trading here you can increase your bitcoins but it needs trading skills and much patience.You can get bitcoins as your salary but now there is no company giving bitcoins as salary but in future it will happen.