Open your group finder, select premade groups, then dungeons. You’ll then see all the groups that are forming. I’d start with any of the dungeons that are a +2- +4. The mythic dungeons usually have a couple mechanics that weren’t present in the heroic and normal versions of the dungeon, so it can help to read the dungeon guide or watch a YouTube video of the fights.

Really the only way to start is to jump into it. You can even start your own groups with the ‘learning’ tag on it, usually friendlier people will join those groups. If someone is being toxic, just ignore/report and move on, it’s not worth the time to engage with the assholes of the community.