The speaker starts right in the middle of the action—in the first person and the present tense: even now, the speaker tastes the mysterious liquor that's going to motivate the entire poem.
The speaker's mysterious drink seems paradoxical, maybe magical. If it's never been brewed, how is the speaker tasting it? The description of the vessels from which the speaker is drinking also suggests that something out of the ordinary is going on here. "Tankards scooped in Pearl" are the kind of drinking mugs one might find in a fairy tale, gorgeous objects made from precious materials. The assonance of "brewed" and "scooped" helps the reader to feel the delectable rarity of the drink and the vessel: those matched, cool /oo/ sounds are like lips puckering to sip a drink, or like the "ooh!" of delight people might make when they taste something delicious.
The next lines only strengthen the reader's sense that the speaker is having an extraordinary experience. The very best wine in the world, made from Rhine valley grapes, can't match the mysterious beverage the speaker's drinking.
Taken together, these lines give the reader the feeling that the speaker is having, not just a good time, but a magically good time. But the sense of mysterious delight in these first lines works almost like the setup for a joke. The speaker is definitely having some serious fun—but as the reader will soon discover, it's the kind of fun that's as down-to-earth as it is transcendent. It's maybe even a little silly.
This opening stanza also makes clear that the speaker will tell readers about this liquor in lilting ballad meter, the rhythm of old songs and old rhymes. Like much of Dickinson's poetry, this poem is made up of quatrains with an ABCB rhyme scheme. That means lines 2 and 3 rhyme, but lines 1 and 2 do not. In this first stanza, though, those rhyme sounds are slant rhymes (another common feature of Dickinson's poetry): "Pearl" doesn't rhyme perfectly with "Alcohol," suggesting the speaker's drunken dizziness.
The meter of the poem, in keeping with its ballad form, consists of alternating lines of iambic tetrameter and iambic trimeter. Recall that an iamb is a poetic foot with an unstressed-stressed, da-DUM, beat pattern; tetrameter just means there are four of these iambs per line, while trimeter means that there are three. Take lines 1-2:
I taste a liquor never brewed –
From Tankards scooped in Pearl –
This is a bouncy, familiar meter that is appropriate for the speaker's light-hearted, joyful tone.