I think with $100 you can buy bitcoin and use for trading, I recommend to trading with Altcoin, you money can increase or multiply if you expert with trading
100$ can only help little bit in day trading. you can start with any good altcoin and earn some profit every day. but for getting good profit you need more money of course. and if you want to be a long term investor in altcoin you need big capital too. at least 1 bitcoin.