Oh, and when can I use the L'? (like l'uomo?)
I found this video very helpful for understanding the usage of definite articles:
l' is used all singular nouns starting with a vowel while lo is used with the masculine nouns starting with s+consonant, gn,pn,ps , x,y,z
See TomSFox' detailed explanation here: http://www.duolingo.com/comment/1055925
L' is the form both "lo" and "la" assume before vowels (with the exception of i+vowel).
Check this link out: http://italian.about.com/od/grammar/a/italian-definite-articles-il-lo.htm