It's using as direct object and with some prepositions. Example
Išao sam u školu.
Pjevamo pjesmu.
Šetamo kroz šumu.

Singular

1) first or a-declination
ending: -a, or - {neuter nouns have -, masculine -a for something alive, or - for something that isn't alive)
čovjek (man, human)-> čovjeka
stol -> stol
nebo -> nebo
2) second or e-declination
ending: -u
žena -> ženu
sluga -> slugu
3) third or i-declination
ending: -
ljubav -> ljubav
laž -> laž

Plural

1) first or a-declination
ending: -e (masculine) -a (neuter)
stol -> stolove
čovjek {irregular, N.pl. is ljudi) -> ljude nebo -> neba
selo -> sela
2) second or e-declination
ending: -e
žena -> žene
ptica -> ptice
sluga -> sluge
3) third or i-declination
ending: -i
ljubav -> ljubavi
noć -> noći