The correct way to fill in the blank in the sentence is:

“We will go to the beach next month.”

This sentence is in the future tense, and the appropriate helping verb to use is “will” to indicate a future action.

The sentence structure for a simple future tense statement is:

Subject + will + verb

In this case, the subject is “We” and the verb is “go”, so the complete sentence in the future tense is:

“We will go to the beach next month.”

Some other examples of future tense sentences:

• She will study for the exam tonight.
• They will travel to Europe this summer.
• I will clean the house tomorrow.
• You will receive your package by the end of the week.

The use of “will” is a common way to express the future tense in English. This helps indicate an action that has not yet occurred, but is planned or expected to happen in the future.