Quite often it's pawns. The advanced pawn provides a target to attack with your f or more commonly h pawn to break open lines towards the king.
You will want to castle queenside if doing this.
Cracking a Castle
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I will add that opening up the centre can really expose the 'fianchettoed' king, although only to a queen seeing as though you have to give up your bishop to achieve this.
I'd say you're attacking much too early. You have no follow-up at all; your development isn't even half-completed, and after the exchange you're effectively attacking with 2 pieces (neither of which can now pose any immediate threat) against 2 defenders and a very solid K/P structure. Aside from that: in your example Black can simply win a pawn (and threaten your Q and/or B) with Nxd4 on moves 4, 5 and 6!
You can't crack a solid castle/fianchetto position quickly (opponent's blunders excepted). Like Scott said, you'll often need to get pawns mobilized, which in turn weakens your own K-side. So you have to get your K to safety and optimize the positions of all your pieces before going for it.
PS: here's one of the most famous and creative attacks on Black's kingside fianchetto, also very instructive because of the skirmishing that precedes the final successful plan:
https://www.youtube.com/watch?v=-iqL2Ald3SU
This is just an example diagram, but it's here to ask a question about how best to attack a similar position. I like to quickly make the B exchange to deprive the structure of the very important piece. But once I've done this, I often struggle for a backup piece to help the Q deliver a killing blow.
What are some general strategies for proceeding at this point? What pieces tend to be useful in making a strike against Black's position?