Know your parts of speech: Nouns, pronouns, verbs, adjectives, adverbs and prepositions are the broad categories of words. A noun is person, place or thing. A pronoun stands in place of a noun (e.g, he, she, him, etc.). A verb is an action word such as cry, shout, walk or run. Adjectives describe or modify a noun. They answer the question, "how many," "what kind," etc. Adverbs do the same thing as adjectives, but with verbs instead of nouns. If someone has brown hair, "brown" is the adjective. If someone is a fast runner, "fast" is the adverb. A preposition is a word that introduces a phrase like "at," "for" or "with."
Know the components of a sentence. A sentence always has a subject and a predicate. Within the predicate is the verb and the parts of the sentence that modify it. The subject is the noun that the verb refers to. The verb and the object (or other modifiers) comprise the predicate. In the sentence, I like to play ball, "I" is the subject, and the rest of it is the predicate. "To play" is the preposition and the verb that comprises the predicate.
Make sure your subject and verb "agree." In each sentence, both the subject and verb must be plural or singular. "The apple is falling far from the tree" is correct. "Apple" and "is falling" are singular. "The apples is falling far from the tree" is incorrect. "Apples" is plural, but "is falling" is the singular form of the predicate. "The apples are falling far from the tree" is the correct plural form.
Don't write fragments. A fragment is an incomplete sentence. Any sentence that does not contain a subject and a predicate (with a verb) is incomplete. "The red bike" contains no action and therefore no verb.
Don't write run-ons. You can recognize a run-on sentence if it has several uses of "and" and "but" separating phrases. Or, if there are complete subject-predicates (sentences) that are not separated by a period.
Write in an active voice. You want to make your subject perform the verb. "Danny threw the pitch" or "I got an F on my English essay" are examples of an active form. Passive form is considered to be nonspecific and undesirable in most cases. "The pitch was thrown" and "An F was received on the English essay" are passive.
Perform further research and practice. After you have worked your way through the above steps, you'll need to practice your writing and eventually do further research on style and usage. The book "The Elements of Style" by E.B White and William Strunk Jr. is a classic on the bare bones of usage and style; it gives the most pertinent essentials.