Whenever I begin to scan a poem in order to ascertain its meter, I think the best place to start is with the words that have more than one syllable, words like Gather and rose-buds, or today and tomorrow, for example. Gather has two syllables, and the first is accented because we say GA-ther, not ga-THER; rose-buds has two syllables, and the first is accented, because we say ROSE-buds, not rose-BUDS. Today has two syllables with the second accented, because we say to-DAY and not TO-day; tomorrow has three syllables with the second one accented because we say to-MOR-row rather than TO-mor-row or to-mor-ROW. One you figure out where the accents naturally fall in the polysyllabic words, you can begin to fill in where the other accents would go. Using the first stanza of the poem, I will put accented syllables, below, in bold font and divide metrical feet from one another with a "|" symbol:

Ga ther | ye rose | buds while | ye may
Old Time | is still | a fly | ing
And this | same flow'r | that smiles | to day
To mor | row will | be dy | ing

From just the first stanza, we can see that the dominant metrical foot is the iamb, a duple foot that consists of two syllables, one unaccented followed by one accented. Further, there are four feet per line; this means that the meter of the poem is iambic tetrameter. The first foot of the first line, consisting of the word gather, is actually a trochee, which is substituted for the iamb. Trochees tend to sound more aggressive or authoritative than iambs, and this makes sense given that the content contains an imperative—a command—to the reader. You will also see that lines two and four are missing their final accented syllables, and this is known as truncation. This also seems to match the content or meaning of the lines, as they specifically discuss how short a time we have to live and how quickly it seems to go; thus, the sense that the lines are short, or feel cut off, seems appropriate.