I am a beginner in Italian language and I am wondering about the 'Lo' in different sentences, like "Lo faccio subito.". What does it mean?
The word lo can be either an article or a pronoun. You can distinguish between the two cases by looking at what follows: if it's a noun (or a pair adjective-noun), then lo is an article; otherwise it's most likely a pronoun.
In this case the next word is faccio, which is a verb and the role is of pronoun. The meaning is “I'll do it at once” where it corresponds to lo. The tense is different: present in Italian, future in English; but it's just how the two languages work. Also in Italian one can say lo farò subito, with essentially the same meaning; using the present gives the idea that the thing will be done really immediately (like in I'm already at it).
There is another big difference: the article lo can be used only before vowels (with elision), z, s+consonant, gn, x, ps and pn (in this case, many people use il, though); otherwise the article should be il.
The pronoun lo is used before any word: it has no alternative form (for masculine singular, of course). The feminine singular form is la, the plural is li.
It is also the equivalent of "I'll do it at once" (the use of future for a decision with immediate effect). For a beginner, I think it would be better to remember that "that" in Italian is normally "quello" or "quella" (and also "che"). "Lo" and "la" should really be the equivalent of "it" or "him" or "her". Of course one could say "I'll do that at once" instead of "I'll do it at once", but that's another story.