Heya, Rose! Check out the Notice in this board. It has a ton of good info about how things work here:
link to a message in this forum.
In general, GMs start games and recruit players to play. Then they go back and forth, posting. Some games use systems like Dungeons and Dragons, and others are freeform games. There’s a diceroller for games that use dice rolls.
GMs post new games in Wanted - Players. Take a look there to see if anything interests you. It you’re looking for a game and don’t see anything you like, you can request a game in Wanted - GMs or perhaps start your own.