I assume you either mean just online or you're not from the United States, where even a part-time job will get you more than $100 a week. For online, the surest way to make $100 is to do what Akuma23 said and write articles. If you treat it like a job and work 8 hours a day, you can easily write 40 articles in a week. The thing is, these are one-time payment deals. If you want to make $100 every week, I suggest starting your own website and then selling stuff through affiliate marketing. It's not easy, and you have to treat it like a job as well and spend many hours building and promoting your site, and even then you aren't guaranteed to make a lot of money.