The second declension isn't too hard. It is a lot like the first declension in many ways. You can tell that a word is in the second declension when its genitive singular form ends in -i.

Words in second declension are either masculine or neuter. The only exceptions are words for trees and names of places. The masculine words have endings different from the neuter words.

The endings for MASCULINE second declension words are:

​	Singular	Plural
Nominative	-us (-er or -r in some nouns)*	-i
Genitive	-i	-orum
Dative	-o	-is
Accusative	-um	-os
Ablative	-o	-is

*Most common examples: vir, viri (man), puer, pueri (boy), ager, agri (field), magister, magistri (male teacher)
! The second declension vocative is weird - see the page Vocative case for more on that.

The endings for NEUTER second declension words are:

​	Singular	Plural
Nominative	-um	-a
Genitive	-i	-orum
Dative	-o	-is
Accusative	-um	-a
Ablative	-o	-is

As you probably notice, the only differences are that the nominative singular ends in -um, and the nominative and accusative plural end in -a. This follows the rules of neuter nouns - nominative and accusative forms are always the same, and nominative and accusative plural forms always end in -a.

Here are some examples of second declension words declined.

amicus, amici ♂: friend (male)

​	Singular	Plural
Nominative	amicus	amici
Genitive	amici	amicorum
Dative	amico	amicis
Accusative	amicum	amicos
Ablative	amico	amicis

oppidum, oppidi ⚲ : town

​	Singular	Plural
Nominative	oppidum	oppida
Genitive	oppidi	oppidorum
Dative	oppido	oppidis
Accusative	oppidum	oppida
Ablative	oppido	oppidis