All nouns have a case depending on what function they serve in the sentence. Although cases may make learning new words difficult, they actually simplify the word order, because the position of words in a sentence becomes not as crucial. As said earlier, there are seven cases in Lithuanian:

• Nominative (vardininkas)
• Genitive (kilmininkas)
• Dative (naudininkas)
• Accusative (galininkas)
• Instrumental (įnagininkas)
• Locative (vietininkas)
• Vocative (šauksmininkas)

The cases answer to these questions:

N. Kas? (Who, what?) Kas čia yra? Čia yra pieštukas. What is here? Here is a pencil.
G. Ko? (Whose?) Ko tau reikia? Man reikia pieštuko. What do you need? I need a pencil.
D. Kam? (To or for whom?) Kam reikalingas trintukas? Trintukas reikalingas pieštukui. What is a rubber needed for? The rubber is needed for a pencil.
A. Ką? (Who? - object) Ką gausi? Gausiu pieštuką. What will you get? I'll get a pencil.
I. Kuo? (With whom, with what?) Kuo rašai? Rašau pieštuku. With what are you writing? I'm writing with a pencil.
L. Kur? (Where?) Kur yra šerdelė? Šerdelė yra pieštuke. Where is the cartridge? The cartridge is in the pencil.
V. ! - used to address. Ei, pieštuk! Hey, pencil!

Notice how the word 'pieštukas' (pencil) changes in each case.

N. Kas? Pieštukas.
G. Ko? Pieštuko.
D. Kam? Pieštukui.
A. Ką? Pieštuką.
I. Kuo? Pieštuku.
L. Kur? Pieštuke.
V. Pieštuk!

Let's get to know each of the cases.

Nominative (vardininkas)

This case indicates the subject of a sentence. It is the form that is found in dictionaries. For example, draugas (friend, m); draugė (friend, f).
Mano draugas eina namo. My friend is going home.

Genitive (kilmininkas)

This case indicates possession or relationship. It is the equivalent of English's genitive 's. For example, my friend's book. In Lithuanian it would be mano draugo knyga or mano draugės knyga.
Ten jo draugės namai. Over there is his friend's home.

Dative (naudininkas)

This case indicates indirect objects. Draugui (m), draugei (f).
Tavo draugui reikia eiti namo. It is needed for your friend to go home.

Accusative (galininkas)

This case indicates direct objects. Note that in singular words it will most likely have a nasal vowel (ą, ę, į, ų), therefore it is easy to recognize accusative nouns: draugą (m), draugę (f), medį (a tree), cukrų (sugar).
Accusative case is easy to confuse with the dative case, especially if your native language doesn't have these cases. English, however, possesses direct and indirect objects: I called my friend (direct object); I gave my friend a call (indirect object).
Aš turiu draugą. I have a friend (direct object).
Aš skaitau draugui. I'm reading to a friend (indirect object).

Instrumental (įnagininkas)

This case indicates the method (instrument) by which some action is accomplished. It is equivalent to English 'with', 'by'.
Aš rašau pieštuku. I write with a pencil.
Pasitikiu draugu. I trust a friend.

Locative (vietininkas)

This case indicates a location or where something is. Lovoje - in bed; medyje - in a tree; namuose - at home.
Sometimes it can indicate time: pradžioje - at the beginning.
Mūsų draugai guli lovoje. Our friends are lying on the bed.

Vocative (šauksmininkas)

This case is used to address someone or something. For example: Drauge, ateik čia. Friend, come here.

Now that you more or less know what cases are, you may get to know the five declensions.