In is usually used for small places (in tasca = in [one's] pocket, nel forno = in the oven, in macchina = in the car etc.), large open spots (nel parco = in/at the park, nel bosco = in the woods, in mare = at sea etc.) and countries (in Francia, in Germania, in Spagna).
A is used for towns (a Milano, a Parigi, a New York) and nouns of building with a specific function (a casa = at home, a scuola = at school, all'ospedale, al ristorante, alla stazione, alla fermata del bus = at the bus stop, al cimitero etc.).
This supposing you're talking about being somewhere. Going somewhere has different rules, and all rules have exceptions.
Anyway for more details check this site out
Nel is in + il
Nello is in + lo
In Italian, l’articolo determinativo has different forms according to the gender, number, and first letter of the noun or adjective it precedes. There are some guidelines that help you to determine what article to use.
For singular nouns:
lo-LO: is used for all masculine nouns beginning with s+consonant or z.
il-IL: is used for all masculine nouns starting in a consonant, except the case where you use "lo".
la-LA: is used with feminine nouns starting with any consonant
l’-L’: This is the elision of lo or la is used before masculine or femenine nouns beginning with a vowel.
lo zucchero » the sugar
il vecchio zio » the old uncle
lo zio » the uncle
l'impatto » the impact (masculine)
il castello » the castle
l'acqua » the water (feminine)
la scatola » the box
For plural nouns:
gli-GLI: It is used whith masculine nouns starting with vowels, the consonant z , cluster gn or clusters made of s+consonant.
i-I: It is used whith masculine nouns starting with consonants which do not belong to the previous case:
le-LE: It is used before any plural feminine noun
gli artisti » the artists
gli elefanti » the elephants
le ali » the wings
i ragazzi »the boys
i cavalli » the horses
gli stessi ragazzi » the same boys
whence adding in to the articles you get
il = nel
lo = nello
l' = nell'
le = nelle
gli = negli
i = nei
same for di: del/dello/dell'/delle/degli/dei