The New Testament was written in Greek (with the exception of a few words such as "maranatha" and "tetelestai," which are Aramaic). It is koinh or "common" Greek, which is slightly different from the formal Greek that would have been used in legal documents.
But don't think that the New Testament was written in some kind of slang "street language." New Testament Greek is highly structured. For instance, nouns "decline," similar to the way that verbs conjugate. Nouns have endings that indicate what they're doing in the sentence, that is, whether the noun is doing the acting, or being acted upon, or being addressed ("Lord, save me!"), etc. It's called the "case" of the noun nominative, vocative, accusative, genitive, or dative.
The "declension" of logoV looks like this: logoV, logh, logon, logou, logw. And that's just the singular.
This makes Greek easier to translate (sometimes). In an English sentence, the order of the words is very important; "The dog bit the man" is not the same as "The man bit the dog." In Greek, it wouldn't make any difference; "dog" would be nominative and "man" would be accusative, so that you'd know who was doing the biting (and who was receiving the bite), no matter where the words appeared in the sentence.
It is a richer language than English, which is why a single Greek word may require three or four English words to translate it. An example: the single word aceiropoihton in II Corinthians 5:1 is properly translated "not made by hands."
On the other hand, the NIV translates the Greek expression apelqousai [from apercomai] opisw sarkoV eteraV (four Greek words) with the single English word "perversion" (Jude 7). The Greek expression literally means "followed after other flesh."
There's a philosophy about translation of Greek (or any other language) that holds that one should be careful not to "microscope in" on a single word; when one translates, one doesn't translate just one word, nor does one translate each word in a sentence individually; it's always a series of words acting together to express something. Context is extremely important.
We don't have any of the "original manuscripts" of the New Testament (known as "autographs"), but we have handwritten copies, some of which were made shortly after the originals were written. They have no punctuation, and occasionally translators have to make a guess as to where the sentences end.
The oldest NT manuscript (fragment) we have dates back to ca. 125 CE. It's four verses from John's gospel.
We know of about 5,000 NT manuscripts, some of them dating back to the Second Century CE. There is reason to believe that some later-dated manuscripts are more reliable than the earlier ones, where variations show up; but this is a matter for the "textual criticism" experts.
Not one of the 5,000 manuscripts is exactly the same as another. No two manuscripts of, for instance, Paul's letter to the Ephesians are word-for-word copies of each other.
There are 138,020 words, more or less, in the Greek New Testament, depending on whether or not you accept the Pericope Adulterae (John 7:53-8:11), the Johannine Comma (I John 5:7-8), and a few other doubtful passages such as the second half of Romans 8:1 ("who do not walk after the flesh but after the spirit," which doesn't show up in most modern translations) and the snake-handling section of Mark's gospel (Mark 16:9-20).
But none of the textual variations is substantive. It's not as if one manuscript says "Jesus is Lord" and another one reads "Jesus is Satan."
The Greek alphabet is fairly easy to learn, that is, you can learn to read Greek (sound out the words) quickly. Many of the letters are pronounced just like their English counterparts. I once taught a group of high school kids to read Greek in 45 minutes. They didn't know the translation, of course, but they could look at the New Testament (the real thing) and sound out the words. Which is where everyone starts who wants to read the Real Bible.
I took a class in New Testament Greek that lasted two years. When the class began, 120 people were enrolled; two years later (on the very last day of class), there were only six of us.
Our teacher was a local pastor named Dickson; he's the most intelligent man I ever met. He could talk to God in God's own languages. Dr. Dickson didn't charge any tuition; we only had to buy the books.
I suggest you learn as much as you can of New Testament Greek, and then buy (1) a Greek/NIV interlinear New Testament [I got mine at Half Price Books for $15] and (2) an Analytical Greek New Testament. It's a thing of beauty. It parses every single word in the New Testament, that is, it shows you whether it's a noun, a verb, a passive verb, an articular infinitive, or an adjective acting as a noun.
The serious Greek NT guys sneer at my interlinear Greek/NIV. "Training wheels" they call it.