Hi Allysson! I'm learning Italian too :)

This wiki page helps me a lot. Hopefully it will be of help to you too :) By the way, I believe what you're asking about can be found under section 2, "Inflection of nouns and adjectives"

http://en.wikipedia.org/wiki/Italian_grammar

Cheers!

If you divide nouns in those, which variate at the plural, and those, which don't, you have this "rule of thumb":
Variable names, regardless of their final letter, take the ending -i at the plural, with the exception of the feminine names which end in -a. These ones take the -e ending (ragazza -> ragazze, porta -> porte).
So,
l' attrice -> le attrici ( "actress", this is a feminine noun ending in -e, it variates at the plural and, as its singular doesn't end in -a, it takes -i at the plural),
il matematico -> i matematici (here, the last "c" is pronounced as "k" in the singular and as "ch", "č" at the plural),
il libro -> i libri,
la mano -> le mani (and this is a feminine noun which ends in -o! also: la radio, la moto and others)
il pilota (masculine, it ends in -a but is a masculine name, so not an exception to the rule) -> i piloti 
la scatola (box) -> le scatole (again, a F. noun which ends in -a, so -e at the plural).

The names, which don't change... don't change XD and there are masculine and feminine nouns in this group.
In general, many nouns that end in -i are F. (analisi, prognosi, oasi...), with some exceptions, and these nouns don't change.
Foreign words don't change at the plural, even if a lot of people here do change them :(

Exceptions: some feminine nouns which end in -a at the singular take the ending -i at the plural.

L'arma -> le armi (weapon) 
L'ala -> le ali (wing)
but also -e is permitted, although it sounds "old", "le arme", "le ale" and other similar forms are used in poetry for example.

That's pretty much correct, even though there are exceptions and the rules get a little bit more complicate for certain endings and for compound names.

Standard:

il piatto (M) -> i piatti (MP)

la ragazza (F) -> le ragazze (FP)

Exceptions (masculine names in -a, -e)

il padre (M) -> i padri (MP)

il poeta (M) -> i poeti (MP)

Ending modifications:

il cuoco -> i cuochi

il chirurgo -> i chirurghi