Poetry is scanned, that is, read metrically, in units of feet. The basic feet are:
IAMBIC: two-beat, rising rhythm, as in adVANCE
TROCHAIC: two-beat, falling rhythm, as in FALLing
ANAPEST: three-beat, rising rhythm, as in on the SHORE
DACTYLIC: three-beat, falling rhythm, as in FRIGHTening--not very common in English
SPONDAIC: two-beat, with both beats stressed--uncommon.

English is a stressed language; that is, meaning is conveyed in part by the contrasting accents of syllables, as in "dessert" versus "desert". Some theories of poetry consider number of syllables as well ("accentual syllabic" scansion), but for most purposes it seems best to consider the number and placement of stressed or accented syllables.
The essence of poetry is variance within repetition, so you should look for broad patterns, and whenever possible try to interpret seeming exceptions as variations of an underlying rhythm rather than entirely new beginnings. (For example, a poem may be basically trochaic tetrameter with iambic passages, or iambic tetrameter with anapestic variations in the second and third lines of each quatrain.) Of course there may be differences in how individual readers will inflect a series of words or a line, especially in the apportionment of SECONDARY STRESSES, but most of the poems we read have a clear identifiable metrical form.
When learning to scan, read a couple lines out loud until you start to hear a consistent pattern. After you have the basic pattern, note how variations are used to extend the author's theme. Variations are more often found at the beginning of stanzas and lines, so that it might be useful to find the pattern in later lines, then return to puzzle over the beginning.

Iambic pentameter: (Christina Rossetti)
A queen in opal or in ruby dress,
A nameless girl in freshest summer greens,
A saint, an angel;--every canvass means
The same one meaning, neither more nor less.

Trochaic and iambic tetrameter: (Alfred Tennyson)
Who is this? and what is here?
And in the lighted palace near
Died the sound of royal cheer;
And they crossed themselves for fear,
All the knights at Camelot:
But Lancelot mused a little space;
He said, “She has a lovely face;
God in his mercy lend her grace,
The Lady of Shalott.”

dactylic and trochaic dimeter:

Half a league, half a league,
Half a league onward,
All in the valley of Death
Rode the six hundred.
“Forward the Light Brigade!
Charge for the guns!” he said.
Into the valley of Death,
Rode the six hundred.

Trochaic tetrameter, fourth line iambic: (William Blake)
Tyger! Tyger! burning bright
In the forests of the night,
What immortal hand or eye
Dare frame thy fearful symmetry?

Mingled trochaic, iambic and dactylic: (Algernon Swinburne)
I will go back to the great sweet mother,
Mother and lover of men, the sea.
I will go down to her, I and none other,
Close with her, kiss her, and mix her with me;
Cling to her, strive with her, hold her fast
O fair white mother, in days long past
Born without sister, born without brother,
Set free my soul as thy soul is free.

Anapestic and iambic hextameter:
I have lived long enough, having seen one thing, that love hath an end;
Goddess and maiden and queen, be near me now and befriend.
Thou art more than the day or the morrow, the seasons that laugh or that weep;
For these give joy and sorrow; but thou, Proserpina, sleep.

Iambic tetrameter: (Matthew Arnold)
Who ordered that their longing’s fire
Should be, as soon as kindled, cooled?
Who renders vain their deep desire?--
A God, a God their severance ruled!
And bade betwixt their shores to be
The unplumbed, salt, estranging sea.

Iambic pentameter: (William Morris)
Her voice was low at first, being full of tears,
But as it cleared, it grew full loud and shrill,
Growing a windy shriek in all men’s ears, . . . .

Anapestic and iambic hexameter: (Amy Levy)

I have neither a voice nor hands, nor any friend nor a foe:

I am I--just a Pulse of Pain--I am I, that is all I know.

For life, and the sickness of life, and death, and desire to die--

They have passed away like the smoke; there is nothing but Pain and I.

FREE VERSE: highly variable line lengths, no rhyme, carefully balanced clauses, repetition, alliteration, and assonance. Rare in Victorian period; found in sections of Tennyson’s “Maud.” A blending of conventional and free verse is found in Christina Rossetti’s “Winter: My Secret.”

The numbers of feet in a line are counted thus: monometer, dimeter, trimeter, tetrameter, pentameter, hexameter, septameter, and octameter. A common nineteenth century form was the hymn meter, iambic tetrameter and trimeter quatrains. A sonnet, of course, contains fourteen lines of iambic pentameter, divided after the eighth line. Blank verse, or unrhymed iambic pentameter, carried associations of Shakespeare's dramas and Milton's Paradise Lost.

Other lines are enjambed, so that it makes more sense to scan across lines--that is, apparent breaks in the meter are signs that pauses are determined by punctuation rather than line ends.

Iambic and anapestic are rising rhythms and trochaic and dactylic are falling rhythms. Most English sentences have a rising pattern, and thus iambic and iambic-anapestic rhythms are common, and often trochaic lines revert to iambic from time to time, forming a syncopated effect. One shouldn't assume that irregularity is a fault; poets take pride in subtle and meaningful variation, but one has to know the pattern in order to understand how it is varied.

This is only half of the story--it's also important to consider stanza length. In addition to sonnets (8-6), ballads (4-3-4-3), abcb) and terza rima (triplets aba, bcb, etc.), stanzas may be of variable lengths (5 lines-12 lines or more), with the lines varied to create emphasis or surprise. Spenserian stanzas have 9 lines, with an extra beat in line 9, and rhyme royal has 7 lines, with the final two lines rhymed -- ababbcc).

Many poems have stanzas of varying lengths and with different metrical patterns to fit the meaning. A poem of varied stanza patterns, line lengths or meters may be called an ode.