Google ta3reeb (Arabic transliteration) offers an option for converting Roman characters to Arabic characters. This lets you type Arabic words phonetically in English script and still have them appear correctly in Arabic. Note that this is not the same as translation -- it is the sound of the words that are converted from one alphabet to the other, not their meaning. For example, typing "ta3reeb" transliterates into Arabic as: "تعريب".
Because of the nature of this system, there is no single "correct" way to write an Arabic character (e.g., "ج" might be transliterated as "j" or "g"). Normally, Arabic letters are represented by a letter (or a combination of letters) that are phonetically equivalent (or nearly equivalent) in English. For example, "ب" can be well represented as "b", "ت" as "t", "ث" as "th", and some may represent "ط" as "t". Arabic letters that do not have a close phonetic approximate in the Latin alphabet are often represented using numerals, or numerals followed by an apostrophe(`) or a single quote ('). These numerals are chosen so that they graphically approximate the corresponding Arabic letters (e.g., "ع" is represented using the numeral "3"). The following table summarizes those special numerals: