Grammar: 1st-2nd Declension Nouns
In general, the 1st declension consists of feminine nouns that end in -a. There are a few masculine nouns in this declension as well, such as poeta and nauta.
The 2nd declension consists of masculine nouns that end in -us and neuter nouns that end in -um. There are also some masculine nouns that end in -r (instead of the expected -rus): puer, ager, etc.
Make sure that you are perfectly familiar with these endings:
Note that the vocative is not included here. It is not, properly speaking, a case - and the only distinctive form of the vocative is for the masculine nouns and adjectives of the second declension: bone domine!