Lithuanian has quite a lot of endings. In order not to get confused, they're divided into five groups - five declensions. The first and the fourth declensions are only masculine, whereas the second, third and fifth belong to both genders.

First declension

As mentioned, the first declension is masculine only.

Examples: vyras, vyrai (a man); kelias, keliai (a road); žodis, žodžiai (a word); arklys, arkliai (a horse).
Those nouns that end in -dis and -tis change d into dž and t into č in singular G., D. and I. and in all plural cases. Take briedis (a moose) for example: briedis - briedžio - briedžiui - briedžiu - briedžiai. Vokietis (a German) - vokiečio - vokiečiui - vokiečiu - vokiečiai.
This is a very common declension, so when you see a word ending in -as, you can decline it using the prior scheme.

Second declension

This one is most likely feminine, but some masculine words belong to it too.

Feminine examples: diena, dienos (a day); giria, girios (a forest); eglė, eglės (a christmas-tree); marti, marčios (daughter-in-law). Masculine example: dėdė, dėdės (an uncle).

Example of the appellative: valkata, valkatos (a tramp). Read the first lines of what is appellative.

Nouns that end in -ti change the t into č in all other cases: marti - marčios - marčiai - marčią. Nouns that end in -tė or -dė change from t into č and from d into dž, i. e. eglutė - eglučių (a little christmas-tree), kėdė - kėdžių (a chair).

Third declension

The third declension is also for both genders.

Good examples are: masculine vagis, vagys (a thief); feminine viltis, viltys (a hope). As you see from the scheme, both words' endings differ in the singular dative: vagis - vagiui, viltis - vilčiai.

In the plural genitive case you can see a palatalizing (softening) letter i. It disappears in several exceptional words: ausis - ausų (an ear), dantis - dantų (a tooth).

In nouns that end in -tis and -dis, t changes into č and d into dž in singular dative and plural genitive (except for a few words). For example: mintis - minčiai - minčių, širdis - širdžiai - širdžių.

Fourth declension

This is another masculine-only declension.

Examples: sūnus, sūnūs (a son); stalčius, stalčiai (a drawer). Notice that all the nouns ending in -ius are declined in plural exactly as in first declension's plural. View it again.

Fifth declension

This is the smallest declension of all the five. There are only two feminine words that belong to this declension: sesuo (sister) and duktė (daughter). The nominative case has a short form -uo and -ė in duktė. In other cases, en and er are added in masculine and feminine words respectively.

Some masculine examples: vanduo, vandenys (water); liemuo, liemenys (waist). As mentioned, there are only two feminine words of the fifth declension: sesuo, G. sesers, seserys; duktė, G. dukters, dukterys.

There, we have overviewed the five declensions. If you feel that you more or less understand everything, you may pick some more grammar from this lesson list.