Look at your sentence and see how many and what types of clauses it contains. If it has only one independent clause and no subordinate clauses, it is a simple sentence. It will contain a subject and verb (The boy ran), but may also contain other parts of speech, such as adjectives and adverbs (The small boy ran quickly). A simple sentence can even contain prepositional phrases (The small boy with the green shirt ran quickly down the hill). The length of the sentence is not the issue; as long as there are no subordinate clauses or additional independent clauses, you have a simple sentence.
If when you examine your sentence you find that there are two or more independent clauses joined by a comma, semicolon, or a conjunction word such as "and" or "but," you have a compound sentence (The small boy ran quickly but he tired easily). There will not be any subordinate clauses in a compound sentence.
If your sentence contains an independent clause and a subordinate clause, you have a complex sentence. The subordinate clause will begin with a subordinate conjunction (such as "after," "as," or "because") or a relative pronoun (such as "that," "who," or "which") and cannot stand alone as a complete thought (The small boy ran quickly because a storm was approaching).
If your sentence contains two or more independent clauses joined by a conjunction, plus one or more subordinate clauses, you have a compound-complex sentence (The small boy who lives in my neighborhood ran quickly but he tired easily.)