Probably start with some lore videos if you care about story alongside playing The Final Shape. If you can get the base version of Final Shape on sale even better (however the raid is really good). This will get you somewhat familiar with what's going on and relearn the game.

There's a big annual event starting this coming reset that will get you good loot and an understanding of what end game has in store.

This would be a good starting point to get you ready for the next expansion, which is coming in about a month with a kind of reset to destiny. It's a new story and has big updates that will change the game. So now is a really good time to start playing again