I think you better keep on reading on this forum for to find out what to do of how to 100usd in BTC. Bitcointalk was very informative regarding about the question that you wanna know the answer.
That is given that many information he can get to make his $100 profitable. And I want to suggest that buy some alt coins that has potential in the future.
Don't just buy alt coins that do others predicting that it is going to pump in the future without proper knowledge and proof.
Just learn the background and devs of that alt coins so that you know how you are going to get profit with it.
Yes investing in altcoin is the right thing to do as if price pumps after that then you can make good profits with that and if you can afford to take higher risk then go for gambling.