Yellow mob is when the mob's name appears yellow to you when you click on them.
I normally solo those, because they're easy for me xD
Once you hit 30, try out the battlegrounds :D It's an awesome place to get exp and good drops.
Also, it's still a good idea to go where your quests tell you to go. That's why I left MT really late, because my quests told me to kill wolves and stuff >__>;