The poem begins with a general statement about humanity. According to the speaker, people expect too much out of "the future." We are "Always too eager" for what's coming next (as if making the impatient demand in the title: "Next, Please"). By implication, we're not nearly focused enough on what we already have. As a result, the speaker says, we "Pick up bad habits of expectancy": basically, a habit of false or exaggerated hope.
And exaggerated hope is easy to conjure up, because "Something is always approaching" on the horizon of our lives. We can always tell ourselves that some stroke of good fortune is on its way (whether a new job, new love, or something of the kind). As soon as we imagine this "approaching" windfall, we fixate on it, telling ourselves "Till then" each and "every day." In effect, "we say" to ourselves: Just wait until X happens: then you'll be happy. But already, it's clear from the speaker's tone and opening statement that this expectation will prove ironic.
These opening lines also establish the poem's form, which will stay consistent throughout. The stanza is a quatrain with an AABB rhyme scheme, and it shifts from iambic pentameter in the first three lines to iambic dimeter or trimeter in the fourth. An iamb is a poetic foot with two syllables arranaged in an unstressed-stressed pattern (da-DUM); pentameter means there are five feet per line while dimeter/trimeter means there are just two/three.
The meter isn't always exact. For example, line 1 here begins with a trochee (the opposite of an iamb):
Always | too ea- | ger for | the fu- | ture, w
Still, the iambic meter lends the poem a relatively steady, familiar rhythm. The shorter fourth line then seems to bring the stanza up short, as if defying or falling short of the reader's expectations:
Till then | we say,
Through this effect, the form cleverly evokes the poem's main theme: disappointment.