When you are asked to "scan" a poem, you are being asked to parse it and identify its stresses and, therefore, its meter. The process of doing this is referred to as "scansion." You are being asked to mark the syllables and feet because this will help you sound out the meter of the poem in your head.

A "foot" in poetic terms means a stressed syllable. So, it doesn't always matter exactly how many syllables are in a line of poetry, provided that the number of stressed syllables fits into the meter. Let's look at the first line:

When old age shall this generation waste

We have ten actual syllables here, but that's not the same as the number of feet. If we were to identify the usual points of stress in this line, it would be something like this:

When OLD age SHALL this GENerATion WASTE

That's five feet—you may be familiar with the term for a line of poetry that has five stresses, as this is the dominant meter in Shakespeare: iambic pentameter. If you practice this kind of sounding-out across the other lines, you should be able to find one in which you don't feel it is natural to emphasize five beats. The poem may still be in iambic pentameter as a whole—an occasional line that varies is commonly found in poetry.

When indicating the rhyme scheme, you simply need to look at the word at the end of each line—we can see there's an ABA pattern, but then the next line deviates—so we call that C. The final line rhymes with the second line, so we find an ABACB pattern.

Scanning poetry means that a person is marking out the rhythm and meter of a particular poem or line within a poem. Most poets try to follow a repetition of stressed and unstressed syllables within each line of poetry. That repetition gives poems rhythm. Each unit of rhythm is called a "foot."

Typically a person will scan a line of poetry by placing marks above each stressed and unstressed syllable. A "-" means unstressed, and a "/" means stressed. That's difficult to do on a computer, so I will use bold for stressed and normal for unstressed. I also can't do a short vertical line on the computer, so I will use a "/" to mark out the feet.

When old' / age shall' / this gen' / -er -a' / -tion waste',
Thou shalt' / re -main', / in midst' / of o' / -ther woe'
Than ours', / a friend' / to man,' / to whom' / thou sayst,'
"BEAU' -ty / is truth', / truth beau' / -ty"---that' / is all'
Ye know' / on earth,' / and all' / ye need' / to know.'

The above rhythm and meter is iambic pentameter. The unstressed/stressed foot is an iamb. There are five of those per line, which is why it is pentameter. Notice that the fourth line in the above section slightly deviates from the iambic rhythm. The first two syllables are stressed/unstressed. That is the trochaic foot.

The rhyme scheme of the above section is ABACB. Lines 1 and 3 rhyme and lines 2 and 5 rhyme; however, the above lines are part of an entire stanza.

O Attic shape! Fair attitude! with brede 

         Of marble men and maidens overwrought, 

With forest branches and the trodden weed; 

         Thou, silent form, dost tease us out of thought 

As doth eternity: Cold Pastoral! 

         When old age shall this generation waste, 

                Thou shalt remain, in midst of other woe 

Than ours, a friend to man, to whom thou say'st, 

         "Beauty is truth, truth beauty,—that is all 

                Ye know on earth, and all ye need to know.