To say because, you must use one of these constructions:

nes
dėl to, kad
todėl, kad

Nes is the most common way to say because, since it's the shortest.

Jis jos nekenčia, nes ji perka stalą. He hates her, because she is buying a table.
Ji perka kompiuterį todėl, kad mokykloje ant stalo yra kompiuteris. She is buying a computer, because there is a computer on the table at school.
Mokinys eina į mokyklą dėl to, kad reikia. The pupil goes to school because it is necessary.

There is also another way to say because or since:

kadangi

It is not completely the same as nes, although it is often used as nes.

The difference is that usually kadangi is used in the first sentence clause:

Kadangi ji perka stalą, jis jos nekenčia. Since she is buying a table, he hates her.
Kadangi mokykloje ant stalo yra televizorius, ji perka televizorių. Since there is a computer on the table at school, she is buying a computer.

But it is possible to use it as nes - in the second clause:

Mokinys eina į mokyklą, kadangi reikia. The pupil goes to school since it is necessary.

Nes is never used in the first clause!