It doesn't. The "bir" is simply the number "1". "Bir elma" is "one apple", which you can also say as "an apple".
One thing to note about plurals in Turkish: When you add a number in front, the root does not take a plural suffix. So, unlike in English, where you would say "one apple, two apples, three apples, etc", in Turkish, you would say "one apple, two apple, three apple, etc" (notice the lack of the plural-making -s at the end. So you would say "bir elma, iki elma, üç elma, etc. HOWEVER, you CAN say "apples" by itself (without a number specifier). As in, "the apples on the table" (masadaki elmalar).
In terms of the suffixes for making plural forms, there are two possibilities: you can either use -ler or -lar.
What determines when it's -ler vs -lar? A little rule called "vowel harmony", which you'll pick up later. Basically, in order to avoid the vocal strain caused by constant ups and downs due to switching back and forth between different types of vowels, Turkish has grouped its eight vowels into two groups of four: Aa, Iı, Oo and Uu constitute the "hard vowels". Ee, İi, Öö, and Üü constitute the "soft vowels". In essence, vowel harmony dictates that hard vowels should only be used together and soft vowels should only be used together, and that these two vowel types should not mix together. So a word like "kimono" does not fit vowel harmony rules, because it mixes a hard vowel (o) with a soft vowel (i).
Vowel harmony extends into how word roots interact with their suffixes. Again, vowel harmony dictates that the vowels found in a suffix ought to be in the same vowel family (hard vs soft) as the last vowel of the root. So, for example, "masalar" (tables) is composed of the root "masa" (table) and the plural suffix -lar. The "a"s correspond to each other (i.e. both hard vowels), and the word thus fits the vowel harmony rules. Similarly, the word "eller" (hands) is composed of the root "el" (hand) and the plural suffix -ler. The "e"s correspond to each other (i.e. both soft vowels), and the word thus fits the vowel harmony rules.
So now that you know the basic rule, what would you append to the roots "tür" (kind/type), "kalem" * (pencil), "kitap" * (book), "yol" (road), "deniz" (sea), and "göl" (lake) to make them plural? You would say türler, kalemler, kitaplar, yollar, denizler, and göller, respectively.
*You'll notice that the roots "kalem" and "kitap" themselves do not fit into vowel harmony rules. This is because they are foreign language imports, and vowel harmony does not apply to the stem of loanwords. It does, however, apply to what suffixes may be appended to those stems, as we saw with kalemler and kitaplar.
R's at the end often voiced a little differently in everyday use. Not like in "raw" or " roof" but imagine a dog making the sound "rrrrrrr, woof!" Now take that "r" and use it for a moment. For a very short time. Voila! It almost sound like "ş" letter in Turkish. Alas, it is not necessary to give effort to that. It is something done out of necessity when speaking, given the sounds in Turkish language.
That is certainly NOT a general rule for all Turkish words ending with an 'r'. So don't be misleaded. In spoken Turkish we do that for a few words but they do not have to end with an r. For example: -N'aber? (Proper written form is Ne haber? Meaning literally "what news", used when asking "what's up" to your buddy) -İyidir. Bu akşam bi' (informal spelling for "bir") filme gidiyoruz, geliyo' (in informal language the 'r' sound at the end of present continuous tense suffix -yor drops) musun? (Meaning: Fine. We are going to a movie this evening, are you coming?)
Bir --> (one/a) in Turkish. "The" in English is used for the "article."
The subject definite article "the" The subject definite article "the" does not exist as a word in Turkish. Subjects are understood as being "specific". There is no Turkish word for the subject definite article, only the context tells us when to insert "the" in English:
Çay pahalı. - Tea is expensive.
Çay soğuk. - The tea is cold.
Araba caddede. - The car is in the road.