I and İ (or their lowercase counterparts, ı and i, respectively) are two separate letters in Turkish, and constitute two of the total of eight vowels found in the language. Despite the visual resemblance, these two letters are not accented versions of a single sound, nor do they sound that similar to one another.
The dotted İ/i is the same sound as the "i" in pin, win, sing, etc. Very straightforward.
The undotted I/ı is a sound that is not found in the English alphabet, but it is found in the pronunciation of certain English words. For example, if we were to write the word "Britain" the way it would be pronounced, but only using Turkish letters, it would be "Britın". Another example would be the verb "to pardon". Using Turkish letters to approximate the English pronunciation, this would be spelled "pardın". So the closest approximation is an "uh"-like sound. A last example would be the word "nation", which would become "neyşın". (It's tough to give examples without letting slip some other new material in. You'll notice that I also dropped in the new letter "ş", which has a "sh" sound, as found in "short", which in Turkish would become "şort")
As is the case with all Turkish letters, they are pronounced consistently, i.e. absolutely the same no matter where they appear in a word. This is in contrast to English, for example, where "i" can be "ee" as in "win", or "ai" as in "ireland", among others.
Sound of 'ı': https://en.wikipedia.org/wiki/Close_back_unrounded_vowel
Sound of 'ü': https://en.wikipedia.org/wiki/Close_front_rounded_vowel
'ü' sound may be a little weird but you will understand voice more or less.
Pronounciation in english varies from word to word. Compare the U in "human" vs "full". In Turkish, sounds are constant. U (without dots) is like the U sound in "full", U (with dots) is the same as in "human". The key to master the pronounciations is to learn the alphabet. Once you learn the sounds in turkish alphabet you will never have to wonder how to pronounce a word. All letters are used exactly the way they are pronounced in the alphabet.
"girl" or "kız" is younger than "woman" or "kadın", as in female child. Although in English, sometimes a young woman might be called a girl as in a female. I think that started as a form of flattery that the woman looks so young that she could be mistaken for a girl. Some teenagers can be mistaken for older also, but you wouldn't want to call them women and so if you are not sure that they are actually women you would call them girls. Now even if you are sure, you might say girls as in they are not so old, especially on the dating scene.