The 2nd Declension
The function of a noun in the second declension is the same as in the first; however, a different set of endings is used (i.e., a nominative is still the subject, accusative is still D.O.). A major issue is that nouns cannot change gender. If a noun is first declension, it has to remain first declension. If it is second, it must remain second.
The endings for the second declension are:
To decline a noun, you must remove the -us and add the endings. Here is a what carrus, carri, m. = cart looks like when it is declined.
That's all there is to it.
NOTA BENE: Declension does not equal gender.