With 100$ you can take some shots at 5NL, if you play cash games, but you need to be ready to drop back down to 2NL, if it drops to 60-80$. If the
poker site has 4NL as the lowest limit, then 100$ will be a reasonable starting bankroll for that as well.
If you play tournaments, then several have already mentioned the 1% rule of thump, which is really easy to remember. With 100$ that basically mean 1$ tournaments or below. Maybe you can bend it slightly and play 1,1$ tournaments, if you also mix in some lower buyins.
It does depend on field size though, and if you play 6-9 man SnGs, then 100$ is a fine bankroll for the 1,5$ variety of these, which many
poker sites has. On
pokerstars specifically you could play a mix of mainly the 1$ 45 man and then some 1,5$ 90 man bounty for the sake of variation.