Here's a guide to Classical Latin pronunciation (Ecclesiastical is all weird and wrong), just in case you are ever forced to read something in Latin:

(letter) - (example) (type) (possibly note)
Consonants
B - "b" as in "bed" (voiced bilabial plosive)
C - "k" as in "kitten" (unvoiced velar plosive)
D - "d" as in "dog" (voiced dental plosive)
F - "f" as in "funny" (labialdental fricative)
G - "g" as in "gun" (voiced velar plosive) (origanally written C, until about 300 BCE, since they are both velar plosives and sound similar. Example: in the Roman first name abbreviation C. for Gaius)
H - "h" as in "hand" (glottal fricative)
I - "y" as in "yummie" (palatal lateral approximant) (later written as "J" by some stupid monks in the middle second millenium, which is why you will often see consonant I's written as J's in a lot of modern etymology)
K - "k" as in "kitten" (velar plosive)*
L - "l" as in "lamb" (latteral approximant alveolar)
M - "m" as in "mommy" (bilabial nasal)
N - "n" as in "ninny" (dental nasal)
P - "p" as in "poke" (unvoiced bilabial plosive)
Q - "qu" as in "quake" (It's actually two sounds. The first is "k" and the second is the Latin "v")
R - "r" as in "sinorita" (dental trill)
S - "s" as in "sock" (unvoiced alveolar fricative)
T - "t" as in "tick" (unvoiced alveolar plosive)
V - "w" as in "wash" (voiced labial-velarapproximant) (Ecclesiastically, it is pronounced as an english "v", but not in classical. So, unless you're at church, remember this! The V was turned to a U around 1000 C.E. by the same monks that would change I to J 500 years later)
X - "ks" as in "licks" (It's actually two sounds. The first is "k" and the second is "s")

Double Consonants
BS - "ps" as in "cups"
BT - "pt" as in "apt"
CH - aspirated "c" *
PH - aspirated "p" *
TH - aspirated "t" *

Vowels
Each vowel has a long and short version. The long version is sometimes shown with a macron over the vowel.
Bold: long, Underlined: short
A - Martha
E - A.O.Hell
I - Seasick
O - Logo
U - Two-foot (origanally written "V", but you can always tell whether "V" needs to be a consonant or vowel by place in the word.)
Y - ü, as in über (German) or French u.

Diphthongs
AE - "igh" as in "might"
AU - "ow" as in "cow"
OE - "oy" as in "boy"

*Found only in words directly taken from Greek.

Stress
There are a couple main rules of Latin stress:
1) On the first syllable if the word has two syllables. e.g. ROma, fIdes.
2) On the second syllable (called Penult) from the end of a polysyllabic word, if that is long. e.g.: amIcus, moneAtur.
3) On the third syllable (called Antepenult) if the second from end (Penult) is short. e.g. dOminus, sociAbilis.

(Note that Penult and Antepenult are plain Latin words: ante-paene-ultima "before-the-almost-last" and only become recondite and impressive in the mouths of the professional grammarians.)

A few further details:
1) If an enclitic such as -que -ne -ve is used, the accent falls on the syllable directly before that enclitic.
2) But certain words like itAque are not encliticized, and compounds like bene-fIcio are not really compounds, hence keep the accent of the verb.
3) Second declension nouns like VergIlius keep accent on original place in genitive and vocative, e.g. VergIli, probably in the interest of clarity.
Information on stress taken from http://community.middlebury.edu/~harris/LatinBackground/Stress.Pronunciation.html

How do we know how these letters are pronounced?! Nobody speaks them anymore!
Well, there are many ways linguists have figured out how they are pronounced. First off is the close relation between Italian and Latin, many of the things Latin does, Italian does too. But most importantly, Linguists can tell what the consonants and vowels sound like from Roman poetry. Nearly all the poetry was written in meter. From these many meters, linguists eventually were able to determine how Latin was accented, stressed and generally pronounced. Another important way that we figured out the pronunciation is through translatorations to other languages.