Have a subject for every verb. The subject is the actor in the sentence. The verb is the action taken in a sentence.
Make sure all of your dependent clauses have a completing thought. An independent clause contains a subject and verb and expresses a complete thought. A dependent clause may have a subject and verb, but it does not express a complete thought. Thus, dependent clauses need to be joined with an independent clause. Sentences that begin with after, when, because, before, since, etc., need a completing thought.
Proofread your writing. All writers benefit from proofreading. Make sure that each sentence makes sense when you read it back to yourself.
Read great literature. When you read great literature, you learn how master's of the craft write. Reading great literature also teaches you proper sentence structure and style. By knowing what good writing is, you will recognize the bad.