It depends on your style. There are many good defences against d4.
Most of them stem from either d5 or Nf6.
d5:
- Queen's Gambit Declined
- Queen's Gambit Accepted
- Tarrasch
- Slav
- Semi-Slav
- Chigorin
Nf6:
- Nimzo-Indian
- Queen's Indian
- Bogo-Indian
- King's Indian
- Benoni
- Benko Gambit
- Grünfeld
When I come across D4 as black, I don't really know what to do against it, and I play some random moves that get me into a lost position. I'm normally used to e4 as black, so I don't see d4 in a lot of games, but I need an opening against d4 if I see it again. Any openings I could use?