Where to start?

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.