NOUNS 
In Latin, nouns have different endings depending on their declension. Declensions denote the endings of the Latin word, which in turn denote which part of the sentence they are in. The endings also describe whether the word is singular or plural (this is known as the "number"). The following are known as the "cases" of a declension, and are always listed in the following order:
Nominative = Subject or Predicate Noun 
Genitive = Possession (ie. "of the farmer" or "the farmer's") 
Dative = Indirect Object 
Accusative = Direct Object 
Ablative = Object of Preposition
[Note: If you are unsure of what any of those objects and predicate stuff mean go here for some English grammar practices]


In addition, the gender of a noun is important in determining its adjective (and verb in passive voice). Though some words may have a feminine ending (like agricola) they may still be male and vice-versa. It is probably easiest to learn a word's definition and its gender at the same time. The three genders regardless of declension are masculine, feminine, and neuter.

In order to properly write a Latin noun, go to the genitive singular of the noun, drop the genitive singular ending (-ae for first declension, -i for second declension, etc.), and add the ending appropriate to its place in the sentence. 
(The genitive singular is usually listed after your noun in your vocabulary list)

To properly add your endings, you must first know your endings tables. In Latin, they are always listed with the endings running down nominative, genitive, dative, accusative, and ablative side-by-side their plural ending. Below are listed all of the endings tables you will need for Latin I.
1st Declension Feminine
Case Singular Plural
Nominative a ae
Genitive ae arum
Dative ae is
Accusative am as
Ablative a is


2nd Declension Masculine
Case Singular Plural
Nominative us i
Genitive i orum
Dative o is
Accusative um os
Ablative o is


2nd Declension Neuter
Case Singular Plural
Nominative um a
Genitive i orum
Dative o is
Accusative um a
Ablative o is
3rd Declension Masculine/Feminine
Case Singular Plural
Nominative -- es
Genitive is um
Dative i ibus
Accusative em es
Ablative e ibus


3rd Declension Neuter
Case Singular Plural
Nominative -- a
Genitive is um
Dative i ibus
Accusative -- a
Ablative e ibus


3rd Declension I-Stem Masculine/Feminine
Case Singular Plural
Nominative -- es
Genitive is ium
Dative i ibus
Accusative em es
Ablative e ibus


3rd Declension I-Stem Neuter
Case Singular Plural
Nominative -- ia
Genitive is ium
Dative i ibus
Accusative -- ia
Ablative i ibus