Search for loose Black pieces. Only the rook at a8 is unguarded—but there is an open diagonal leading to it, making it a promising target. To exploit the situation you need a way to attack the rook while also attacking something else. The queen is the classic tool for such a purpose, but here White's queen has no checks to give. So move to another question: what mating threats does White have? A simple way to search for mating threats is by looking for pieces that attack squares next to the enemy king; adding an attack by the queen against such a square often creates a mating threat that is as good as a check for purposes of creating a fork. Here White’s d3 bishop attacks h7, next to Black's king. White’s queen also can attack h7 by moving to e4 or h5; so the queen’s move to either square threatens Black with mate on the next move. Of these two moves the interesting one is Qe4, since it also attacks the loose rook. Naturally you first ask whether the needed square is available and find it is not: e4 is guarded by Black’s knight.
The procedures for handling this sort of problem are well-known to us now. We start by asking whether the knight can be eliminated with an exchange, and it can; 1. BxN, BxB leaves e4 unprotected, after which 2. Qe4 wins the rook after Black spends a move fending off the mate threat.
We will study this way of creating and using mate threats in more detail soon.