"Money" begins by personifying money, giving it a voice that "reproaches" or scolds the speaker:
Quarterly, is it, money reproaches me:
'Why do you let me lie here wastefully?
This personification elevates money from a mere object or concept to a powerful entity capable of passing judgment. It dramatizes money's role in the speaker's life: the way it seems to make demands, and seems disappointed when its demands aren't met.
Money scolds the speaker "Quarterly," or four times a year. This might imply that the speaker gets paid quarterly; more likely, it implies that they receive quarterly financial statements from various accounts. (Two of Larkin's draft titles for the poem were "Bank Statement" and "Financial Statement.") The hedged phrase "is it" (as in "Quarterly, is it") suggests that the speaker doesn't dwell too much on money. They don't keep close track of their overall financial situation.
Money's complaint is that the speaker wastes it—ironically, by saving it, not spending it. It chides: "'Why do you let me lie here wastefully?'" The question has vaguely seductive undertones (as if money is beckoning the speaker towards its lonely bed), setting up the reference to "sex" in the following line. By begging to be spent, and hinting that its purpose is the fulfillment of desires, this personified version of "money" expresses the values of capitalist, consumer culture.
In lines 3-4, money goes on to define its role in human life. The speaker's unspent money represents all they "never had of goods and sex." Money can buy material possessions and consumer "goods," like a nice big TV or flashy watch. It can also help satisfy more primal longings by being exchanged for sex. The speaker clearly hasn't indulged much in either—though, on some level, they may have wanted to. (Larkin's poems often feature similar sentiments.)
Lurking behind money's words is the idea that happiness is transactional. Though the speaker has so far failed to spend as money thinks they should, their situation can be rectified: "You could get them still [i.e., goods and sex] by writing a few cheques." Money makes life sound appealingly effortless: earn your wages, spend them, and be happy. Note, too, the grim rhyme between "sex" in line 3 and "cheques" in line 4. This breezy rhyme pair underscores the logic behind money's suggestion: cheques equal sex. Everything's a transaction, even intimate relationships.
In its opening lines, then, the poem starts a discussion about the true worth of money, and about money's relationship to personal fulfillment. The speaker personifies money as a kind of foil for their own views: they certainly don't agree with money's take on life!