For now, we will cover three pronounciation points: the syllabic 'ん', the little tsu (or double consonants) and long vowels.

The syllabic ん

First, a word on mora. 'Mora' is how Japanese refer to units of fonetic length - or, to put it simply, units of pronounciation. Every character in hiragana and katakana represent one mora. And so does ん, even though it is only one sound. But, it is not always read as N. When the character ん is placed before b, p, f and m, it is pronounced as M. So, the words てんぷら、さんぽ、えんぴつ and しんぶん　are actually pronounced tempura (tempura, a kind of japanese food with shrimp and some other stuff), sanpo (a walk), empitsu (pencil) and shimbun (newspaper) - you can click on the words to hear he pronounciation.

Doubled consonants - little tsu っ

In Japanese language, double consonant sounds are indicated by a small っ　(as opposed to the full sized one - つ) which is put before the syllable whose consonant needs to be doubled. The little っ　means that the following consonant is to be given two syllables' duration (two mora).
The syllables beginning with k, s, t and p can be doubled, in the wollowing manner:

っか-kka　　　　　っき-kki　　　　　っく-kku　　　　　っけ-kke　　　　　っこ-kko
っさ-ssa　　　　　っし-sshi　　　　　っす-ssu　　　　 っせ-sse　　　　　っそ-sso
った-tta　　　　　っち-tchi　　　　　っつ-ttsu　　　　って-tte　　　　　っと-tto
っぱ-ppa　　　　　っぴ-ppi　　　　　っぷ-ppu　　　　　っぺ-ppe　　　　　っぽ-ppo

せっけん-soap
けっこん-marriage
きっぷ-ticket
ざっし-magazine
きって-stamp
らっぱ-trumpet, bugle
さっか-writer
はっせん-eight thousand
あさって-the day after tomorrow

Long vowels

If a syllable is placed before a certain single vowel character (a, i, u, e, o) it's vowel will be read twice it's original length (or two mora).
Or, simply put:

-If you have あ　after a syllable that ends with A (かあ、さあ､たあ．．．）, both vowels are read as one long A.
EXAMPLES :
おかあさん-mother

-If you have い　after a syllable that ends with I (きい、しい、ちい、にい．．．）, both vowels are read as one long I.
EXAMPLES :
おにいさん-elder brother
おじいさん-grandfather

-If you have う　after a syllable that ends with U (くう、すう．．．）, both vowels are read as one long U.
EHAMPLES :
くうき-air
つうしん-correspondence

-If you have え　after a syllable that ends with E (ねえ．．．）, both vowels are read as one long E.
EXAMPLES:
おねえさん-elder sister

-If you have お　after a syllable that ends with O (こお、とお．．．）, both vowels are read as one long O.
EXAMPLES:
とおい-far
こおり-ice

BUT:

-If you have い　after a syllable that ends with E　（けい、せい、てい．．．）, both vowels are read as one long E.
EXAMPLES :
とけい-clock
がくせい-student
There are a few exceptions to this rule, though. The word for English language, えいご, is still read as eigo, no long vowels.

-If you have う　after a syllable that ends with O （とう、こう．．．）, both vowels are read as one long O.
EXAMPLES :
ひこうき
ぼうし