About your double vowels, attention that if you forgot to pronounce it then it's another word. For example:
chizu = map
chiizu = cheese
obasan = aunt
obaasan = grandma
About writing with your computer:
You'll also encounter little tsu っ which is used to mark double consonants.
For example
matte (with two T), both are heard when you speak (
mat' tekudasai), but in hiragana or katakana you can't write a single letter, it's only syllabics. instead you use the little "tsu" in front of the double letter.
To write まって, you can either type "matte" or "ma xtu te" depending on your input method or software.
It's the same with other letters, not only with T.
to make all little letters, you use x+romaji (xtu (xtsu), u, yo, ya, etc.)
Some input method (IME) write kana one by one, so you need to type ma xtu te. Some other IME are memorizing what you type until you press Space bar or select from multiple choices : for example write "kanojo" and select either the hiragana or the kanji.
check Densetsu's topic for specific help