Hi Natsumi welcome to the forums. If you click on the Forums tab at the top then you can scroll through all the various forums listed by subject. You can read those threads, respond to those threads or post new discussion threads within each forum.
Another way to join conversations is to push on the New Posts option. This will show you each post that has received recent activity. It’s a good way to keep track of what goes on in the posts here. Let me know if you have any questions and have fun looking around. xo, j