What is Voice:
Voice is the style you write in, the personality or background of the speaker. It's the character the poet creates while writing the poem, like the point of view. For example, if the speaker wrote in the voice of a small child, their word choice, punctuation, and knowledge/experience would be different than an adult writing a poem. An adult has more overall experience in life, therefore making the view different than the child's.
The atmosphere and personality of the speaker in the poem are some of the things that make the tone. The voice of a poem is made by many different variables, just as the actions of a character and what they say is what creates their character. The voice of the poem is like glasses that you're creating for the reader to put on and see the world through. The speaker in the poem may be afraid of going near fire, and is describing it. Someone else could write a poem about how they find the fire inviting and friendly. This is because not everyone thinks the same, but the point is, the two people are still looking at a fire. They just /see/ it differently. Writing with voice is making those glasses to fit what you want to write. You're creating the viewpoint and what the speaker sees.
What makes Voice:
A lot of different things make up voice: your word choice, punctuation, and the experience of the speaker matter more than you may think. Even though they all feel like small things that you may not usually think about when writing poetry compared to the big and broad variables like imagery or the theme of the poem.
Word choice: The choice of words the speaker uses during a poem differs from person to person and from style of poem. An example is that the word choice of a poem about or with the theme of "Eye for an Eye" would be different from one with a theme of "Never Give Up".
Choosing one synonym of a word over another may not seem like much, but overall it heavily affects the poem. Let's say you decide to have a line that says "My love for you is undying.", we've all heard this cliche before, but it's an example.
Now replace that word with "lust" and it will become "My lust for you is undying". It changes your thought of the line, right? I tend to think of lust as a more physical thing compared to just "love" which is a very broad word. Change "lust" to "endearment", now it's, "My endearment for you is undying.". Endearment feels like a word that is more emotional than physical, and for me, describes a more platonic love. Words have synonyms, and they do mean the same thing, but they mean the same thing in a more precise way rather than the basic word that the synonym sprouted from, here, it's love.
Another way to think of it is that you just turned Level 20 in a game, and in that game, you chose the Warrior class. When you turn Level 20, you get to choose from three different subclasses, all having roots in the same class, but also different. The three choices are Paladin, Barbarian, and Knight. You wouldn't be describing a barbarian by saying that they're a paladin, they're not the exact same. A barbarian is more of a savage warrior while a paladin is thought of to be more of a righteous warrior. You can think of it like that.
With age, the vocabulary and word choice of a speaker change. If they are young, they might be bound to use more current words, and if they are old, they might use more words that are now more unknown or have a sort of formality to them.
Punctuation: As much as it seems like a small detail, punctuation is more noticable in a poem if you look more into it. Does the speaker even use punctuation? This also reaches out to line length and figurative language. If the lines are short, it could be the speaker showing that it's anxious or short-breathed. On the other end of the spectrum, long sentences could indicate that the speaker is rambling on.
If a person is formal, they might use very strict or formal punctuation, and if they're informal, it may be more lax. Whereas they have no punctuation or punctuation that is more nonchalant of what it is doing or how that affects the poem. The punctuation affects how the lines are delivered and how they read. You may not think of it this way, but punctuation is a stylistic choice in poetry, as you can choose whether or not you want it or how you want it, rather than having to follow exact grammar rules.
Knowledge/Experience: The age and experiences of a person can also deeply affect the poem and its voice. If a cowboy and a knight both wrote poems about horses, they would be different because of their different backgrounds and the different lives they lead. A knight could write about riding their horse into battle, while a cowboy could write about a horseback riding race or something of that sort. This is all just an example, but it still gets the point across that it's all about the view of the speaker. Everyone will view things differently, and describe them differently. They will view things differently because everyone has had different experiences and none the complete same.
Finding Your Voice:
There are many factors that contribute to finding the voice of your poem. Are the poems or poetry you're writing to a specific person or directed to a specific audience? Or are you trying to express some sort of bottled up emotion? Are you the one telling the poem, or is it from a different perspective? These questions all lead into finding the voice you want to use.
It takes time to discover your voice. Don't expect to have a strong variety of voices right away. Experimenting around is the best thing to do to try and find how voice affects your reader. It is like throwing things at the wall until something sticks, and that's when you know it's strong. Take inspiration from things that you like or are personal to and use them to climb further up.
Your voices in poetry are the personas or masks you decide to put on, or a vessel for your emotions. It's the narrator that the reader views when reading your poetry. It acts as the tone and sets up the atmosphere of the poem on how the narrator speaks. You wouldn't have the same experience if the poem was the same topic, but with different speakers.
A number of things you can experiment with are punctuation, capitalization, vocabulary, and point of view. The reader usually picks up on things like if you have punctuation or not, and capitalization or not. This may be also sort of an inbetween where you may use a lot of commas, or only have a handful of them. The point of view and vocabulary go hand in hand, as punctuation and capitalization do. Does the speaker have a lazy word choice where they use whatever works and use slang, or are the words carefully and precisely picked out?
This doesn't exactly mean that you don't have a voice at all. You don't find your voice in one moment, rather it changes through time like clay. It starts to have more prominent features that tend to follow with the finding. Even after it still has tiny changes over time and never stays quite the same, it's always there, you just need to bring out the strong features.
Discovering voice may take time and experimenting around with what works and what doesn't, but when you've found it, it blossoms and develops more and more, the more you write. Finding it is a caterpillar becoming a butterfly, the cocoon is the in-between that you have to break through.