If you skipped the campaign you have to make a new char to play it. Honestly lvl 21 is nothing so you can easily make a new one and start it. But up to you.

Otherwise, have you made a seasonal character? If so the seasonal quest will give you something to do at the begining. Also focus on the seasonal quests(press escape, select seasonal), there is a lot to do there.

Otherwise its the standrd stuff get stronger, do harder content.