How do I know if I have to use the one with accent and not the one without accent? Could you provide examples?
It's just a matter of stress. If the stressed syllable is the first one, you don't write the accent. If it's the last one, you do.
"está" is the 3rd. person, singular of the verb "estar". In English: "is".
"esta" is a pronoun, it translates to "this".
Esta casa está inclinada. Esta linea está torcida. Esta mañana está muy bonita.