I love playing with capital letters and punctuation. These are both important ingredients in a poem.
The great thing about writing a poem is you get to choose what you want to do.
Some poets begin every line with a capital letter.
Some poets never begin a line with a capital letter.
Some poets use capital letters in a poem like they do in a story to show the beginning of a sentence.
If you use capitals down the side all the time it can stop the flow of the poem. Nowadays poets are less likely to do this.
Sometimes young poets give a capital letter to a word in the middle of a line. This can make it stand out. Think about why you want to do this.
Think about why you use capitals down the left-hand side. Do they look good? Do they shout a bit at the reader? Do they add to the rhythm? Do they stop the flow?
Be consistent. Once you make a capital-letter choice it helps the reader if you stick with that choice throughout the poem — unless you want to write a poem that misbehaves with punctuation (some poets do! although not that often).
Have fun choosing!