Answer:
There's something you must know.
Step-by-step explanation:
All of the incorrect sentences contain a subject-verb agreement mistake. The subject-verb agreement refers to that the subject and verb must agree in number (singular or plural).
There's pencils in the box should be There are pencils in the box. As the subject is plural, the verb must be in the plural form as well (are + pencils).
There's five marbles on the floor should be There are five marbles on the floor. Once again, there is more than one marble, so the verb has to be in the plural form (are + five marbles).
There's several ways to post something online should be corrected to There are several ways to post something online. Just like all, both, few, many, and some, several goes together with the plural form of a verb, although in colloquial language you can often hear it used with the singular. Are + several is the grammatically correct option.
There's something you must know is an example of correct subject-verb agreement, as the singular form of the verb (is) is used with the singular pronoun something.