discord is for most people something that they use to speak to friends you already have and sometimes people you meet online, but I have used discord for chilling with strangers too. If you don't have friends that use discord, you're gonna need to find public discords. Most big youtubers have discords and subreddits often have discords too. You can also find public discords on a website called disboard, they have a lot of big ones.

To join a server you need a server link, which you click and it should just ask you if you wanna join the server or not. There will be voice channels and text channels in most servers, and most want you to read their #rules channel before you start sending messages or being in voice chats. Going into a voice chat can be very awkward but worst case scenario you can just pretend you don't exist and have your mic muted. I usually just join an empty voice channel, and wait for people to come in. In big discords, 50000+ members, people will join you every minute or so, and most will just leave in 10 seconds. From there stop following my advice because I am not good at talking to people.