There are many ways to make money online - blogging, affiliate programs, selling your own information products etc.

The first step would be to eductae yourself about the different options avaliable.
Secondly, choose a niche that you want to be involved in, based on your interests. Then figure out if it's profitable or not.
Decide how you are going to make money, adsense, affiliate products, your own infoproduct etc.

Best of luck.