Ok, for the LFG there are a number of categories.
1. Living World: this is level 80 only content. When you reach 80 you'll receive an ingame mail and map notifiers telling you where to go.
2. Dungeons: this is level 30-80 content. You won't see these yet since you are only level 20. As you reach each level required to enter a dungeon again you'll receive an ingame mail showing you where to go.
3. Fractals of the Mists: these can be done at any level but are considered difficult for underlevel players. I'm not even sure if it shows to brand new players now - anyone? Anyway, the "scales" refer to how far you've progressed in Fractals. You can do Scale 1-10, but agony kicks in at level 11. You should not do this yet since you cannot get agony resist gear until you are 80 and if you try to join these dungeons now, you will most likely get kicked without a word.
4. Personal Story: you should have received or completed your personal story missions up to level 20 by now. Players will post there if they need help with their story, you generally only see the Zhaitan story there. This is a level 80 dungeon so not for you yet
5. PvP + WvW: no one uses these, we meet up in the mists.
As you level the game will notify you where to go. As you play through you'll get familiar with the events and where things are located. It can be confusing at first, that's why I recommend joining a guild
EDIT - remember you can always ask in party chat when you join a group "This is my first time, where do I go?" or something to that effect. We mostly don't bite, just don't try to join on groups labeled "speed run" or "level 80 EXP only" until you are ready for it.