How do I scan hexameter?

Latin poems are written in a number of forms. Epic, such as Ovid's metamorphoses or Virgil's Aeneid, are written in hexameter. 

When scanning, there are a couple of important terms you need to know:

A syllable is a vowel or a dipthong. It is one sound, that often has consonants on either side of it. 

A foot is made up of several syllables, and in hexameter there will be six of these in a line. 

A dactyl is the basic form of a foot. It consists of one long and two short syllables, and is shown like this - u u and sounds like dum di di

A spondee is a long foot. It consists of two long syllables and is marked up like this - - and sounds dum dum

A caesura is a break in the line. It must come at the end of a word, and commonly is found in the third foot, sometimes in the fourth and occasionally elsewhere in the line. It is marked //

Elision is when the vowel or sometimes a dipthong is not pronounced at the end of a word. When this happens, it is not counted for the purpose of scansion. 

The final two feet of hexameter will always be a dactyl followed by a spondee, so the lines will be like this:

- u u | - u u | - u u | - u u | - u u | - x

Or: - - | - - | - - | - - | - u u | - x

So, let's think about this in terms of some actual Latin. If we take the opening line of the Aeneid as an example:

arma virumque cano, Troiae qui primus ab ori

I sing of arms and the man, who first, from the shores of Troy....

The first thing to do is count the syllables in the line. In this line, there are 15 syllables. We know that the final two feet have to be - u u | - x, so that takes away five syllables, leaving us with ten syllables to divide between four feet. There are then some rules we need to apply to work out the rest. 

A vowel followed by two consonants is long. This means that the second syllable of virumque and the final syllable of cano are both long, so the scansion is like this:

 x x x - x x - x x x | - u u | - x 

Dipthongs are long. This means that the final syllable of Troiae is long, so we can then get to this:

 x x x - x x - x - x | - u u | - x 

The accusative is a short syllable, so the final a in arma is short, so the line is like this:

x u x - x x - x x x | - u u | - x 

 With these bits of information, we can then make the rest of the line fit in and work out where the feet go. 

The caesura falls on the comma, neatly spitting the line in two. 

So, the line scans thus: 

- u u | - u u | - // - | - - | - u u | - x