A bit more detailed than you might have wanted...
In order to understand the function and realization of extended consonants, it's important to understand that the rhythm of Japanese is based on the
mora, a rhythmic unit much like the syllable. Hiragana and Katakana, often mistakenly referred to as syllabaries, are moraic writing systems, where each character represents a single mora. When read aloud, each character is allotted roughly the same amount of time to say. This becomes apparent when a Japanese speaker "spells" out a word in Japanese: they do not use letters, but will say aloud the morae that make up the word (ex: 家族 = か・ぞ・く = ka-zo-ku).
The only exceptions to this are the use of half-width kana to produce sounds like
きゃ,
しゅ, and
じょ, or non-japanese sounds in foreign loan words like
ファミリー or ミルク
ティー. These sounds still consist of a single mora, even though they are written with more than one character.
The double consonants you asked about are marked in kana by the small tsu っ (ッ in katakana). It's proper name is sokuon, but it is commonly simply referred to as 小さいつ (chiisai tsu). This character does not have a sound of its own, but rather marks that the following consonant sound is extended by the length of one mora. It is not a glottal sound; its placement is the exact same as the consonant it modifies. So for, say, "hatto" （ハット, hat）, you simply hold the /t/ sound slightly longer than for "hato" (はと, pigeon).
For plosives like /t/, /d/, /b/, /p/, /k/, and /g/*, It sounds like you pause before you actually release the sound and continue the word. Examples include:
い
っぽ （一歩）
い
っぱん （一般）
ラ
ッキー and
ほ
っかいどう （北海道）.
For fricatives or affricates like /s/, /z/, /ch/, and /j/, the sound is simply lengthened for the length of one mora, like the difference between the /s/ sound in "sam" and "hiss." Whereas plosives include a full stoppage of the airflow, fricatives require you to expel your breath as you make the sound. So these sounds couple with a sokuon simply sound longer, such as:
い
っしょ （一緒）
い
っせい （一世）
は
っさい （八歳）
and so on...
Most likely, the best way for you to learn to distinguish these sounds is to listen to Japanese (podcasts or the like), and practice mimicking these sounds.
*please note that all phonemes are written in the Hepburn system.
Check out
Wiki's article on Sokuon for a bit more info.
That really helped, because I didn't know they were extended with the s and such! thanks 
