Online Business ideas?

I am currently 17 years old, but my dad is around 50, and we kind of want to make some extra cash on the side by having an online business. I've been trying to think of some ideas of what kind of business we can have, but for some reason i just can't come up with one. It can't be too expensive to invest in, due to the fact that we do not have disposable income. Any ideas would be so helpful..Thanks.