1.d4 d5

Not sure what to play after this. Opponents have a hard struggle and a tough game lies ahead. Thoughts? Greatly appreciated.

for white or for black?

I exclusively play 1 d4. you can take a look at my games for an example of opening lines.

as with most bughouse openings there are many tricks/traps in depending on what specific line is played. Get the first 5-8 moves down then keep playing it often and you will learn the different variations.

nc3 is a logical move. You both develop the knight and you open up for e4 on the third move depending on opponent. I think that I often play e3 and slowly develops my stuff instead of going for attack which most does with white. Bughouse is a mixture between aggressiveness and building up a position.

How to play also much depends on what pieces the other table brings you. You surely wants to move the e pawn soon though to both avoid pressure on f2 and for development of f-bishop.

As with chess, d4 is a solid opening for boring people.  Just develop logically. white will typically play on the dark squares on the kingside and black does the same with white squares. don't get too cramped around your king as black.

 I think some people start with 1. Nf3, but i can causally defend. The sac on f7 doesn't mean anything, as long as you have the right skills to defend

 I don't like d4 very much since it's selfish. While you can play whatever you want as white your partner doesn't have such luxary. In most lines black need pawns to improve their position, in d4 d5 lines early pawn exchanges are unlikely. Your partner is pretty much forced to play e5 against e4 almost the only type of position where black can hold themselfs with no pawns and low flow.

As for white d4 d5 London opening without c3 of course is pretty solid