To be able to write grammatically correct sentences in English, first of all, you need to know what is a noun and what is a verb. This is because, all sentences are build up from at least a noun and a verb.
Basically, nouns mean the name of everything in this world (people, animals, things and places). Whereas verbs basically refer to actions.
Nouns: boy, cat, chair, school
Verbs: eat, play, sleep Verb 'to be': am, is, are
In an active sentence, you will start with a noun which is followed by a verb:
Eg: (noun) (verb)
[A cat] [eats] fish.
Eg: (noun) (verb 'to be')
[The chair] [is] big.