Does an acrostic poem have to start with the first word of every line in a poem, or can you use the middle of a sentence?

You can use the middle of a sentence.

Explanation:

An acrostic poem is a type of poetry in which specific letters in each line spell out a certain word or phrase. The most common type of acrostic poem is the one that uses the first letter of each line. However, any letter at any position can be used to form the desired word. When the first and last letters of each line are used, we have what is called a double acrostic.

Let's see some short and simple examples.

- To spell "dog":

Darling friend through night and day

Over the years, you faithfully stayed

Good-bye for now, in Heaven let's play

- To spell "life":

Nothing hurts more than Living

Yet Inevitably we must

Our own mistakes Forgive an'

ForgEt how life's unjust