If you are just going to buy your first cryptocurrency then I will advice you to stsrt it with Bitcoin even if you buy bitcoins worth of $100 is better than investing into cheap altcoins which may not have potential and its really hard to choose which altcoins to invest for a beginner.
Depends actually. With $100 budget that seems to be a little low for purchasing btc at its current price now. I think altcoin is much okay too however like you said its not that easy to pick. But there are ways on how to choose a good altcoin and it could be a major coin or a new sprout altcoin. Some traders betting more on new projects that could possibly gave a huge profits when that tokens pump so much but that is a hard task for a beginner. So better pick a potential major coins that is safe than newer projects.