Literally find someone in game or ask a friend to help you with the capstone dungeon.
Then start by doing open world content like Helltides, whispers, fields of hatred. The reason why is because you won’t be able to do 21+ yet.
However open world is very easy. A NMD 15 is actually harder than open WT4. This will let you start farming ancestral’s though. Once you net a few you will be fine going from there.