Accessible forms of poetry for journaling?

Find a copy of Perrine's Sound and Sense. It's a textbook, so it's a bit more inconvenient to get than a regular book. But it's easily the best introduction guide to learning how to read and understand poetry from scratch. It goes over all the mechanics and tools, with a helpful selection of classic poems to explain the concepts. It's not a very long book either but pretty dense in terms of coverage to help get one started to reading and appreciating poetry.

One of my favourite things about poetry is there really aren't any rules. Some rhyme, others don't. Some have a particular structure, like haikus, others use spacing on a page to emphasise something. To me, a poem is a painting made of words.

I'm currently reading Let The Light In by Lemn Sissay. He did a project to write a 4-line poem every morning for 10 years and compiled the best ones into this book. Here are a couple of my favourites so far:

Day breaks
And a split-yolk sun
Oozes on a lightly toasted
And buttered sky

In the way light talks to a river
In the way a river holds the night beneath
In the way spring calms winter
In this way we should speak

I really enjoy poems that are written for their sound (with rhyme, alliteration, and above all stress patterns), and one big benefit of these is that they can be very short and still strongly read as poetry.

Writing short poetry is really nice because it's simultaneously a challenge that will really get you practicing the craft tightly, and not as much of a time sink as a longer poem. It's like writing a strict 300-word short story instead of a 60,000+ word novel.

One thing I find helpful for writing poetry, particularly with getting the structure right, is to imagine it as lines for a song. Matching it to music can help you grasp the meter and stresses more intuitively, since song lyrics are virtually always written with close attention to structure.

Limericks are also really great for practice because they have such a familiar, regimented structure that it's usually pretty obvious if you've messed up the stress pattern. These can be really helpful as a warm-up or for identifying stresses in words that you're considering for a different poem.

Here are some short examples I've written for a fiction project, which has a lot of excerpts from a character's journal where he often writes poetry. His poems get particularly terse when he's struggling emotionally (they get longer and more florid when he's happy), so my apologies for how dark some of these are:

Steel your thought and still your heart
Lest you rend yourself apart

Black cloud on sere plain
All shadow, no rain

Sweetly sings the knife
That neatly inks my life
In threads scored red upon the floor

In the last one, I used internal rhyme ('sweetly'/'neatly', 'sings'/'inks', 'red'/'thread', 'scored'/'floor') and some alliteration (repeated 's' sounds in 'sweetly'/'sings'/'inks'/'threads'/'scored', and also some 'f' and 'th' that sound similar) to keep the last line sounding cohesive and linked to the previous two lines despite its break from their structure. By using internal rhyme and alliteration, you can get more loosy-goosy with your phrasing, even on a very short poem.

Here's another example — this one using so much internal rhyme and alliteration that it's getting toward tongue-twister territory (I love writing tongue twisters). The internal rhyme and alliteration let me break away from using a standard rhyming structure, but I'm still keeping the stresses highly ordered (you can break more rules on a longer poem, but with a shorter poem, a stricter structure helps with establishing a pattern in just two lines):

She ladled a riddle into my bowl
And fondled my soul like a fiddle

My only real experience with the medium is from school, and thinking back to that time only reminds me of how confused I was while guessing if a foot was stressed or unstressed.

Oooh, I feel that. I've always been more visual that aural, and I remember struggling with meter in class. Not only would I get the stresses of syllables wrong, but I would get confused how many syllables there were. Which seems like a really basic thing—just count them!—but the sounds of the words were too vague and subjective to my ear.

If it's any comfort, abilities can change over time, or at least they did for me. I occasionally write haiku and enjoy it now. Haiku are especially fun because they're short, and because there's an additional constraint that doesn't have to do with the words themselves (namely, that you must find a nature/seasonal metaphor for your topic).

I like idea of using constraints to further my grasp on the thought I'm trying to express

You might look into constrained writing, which is kind of poetry-adjacent, and see if any particular constraints appeal to you. There might be something more approachable than writing traditional verse.

I am not a poet, but if you want some some forms to start that are VERY structured and can work with multiple lengths:

Acrostic poems have the first letter of each line spell out something. You can make a sentence by breaking each word into separate verses, spell one word, make each line a proper line/sentence or be more abstract, etc. Lots of options there.

Diamante/Diamond is just writing it in the shape of a diamond. First and last lines are one word, and the line lengths go from small to wide to small. The content could all be related to some theme, could gradually shift (start with "Hot" and end with "Cold"), you could make it a palindrome poem, etc. Could also combine it with Acrostic to spell a word.

Related to above, concrete/shape poems are writing the lines in the shape of the poem's subject. Looking it up on DuckDuckGo brought up pictures of "Swan and Shadow", spiraling words to make a sun, a key and outlines of shapes rather than fully filling it in. It's obviously a bit more visual and requires some sort of physical subject to depict, but lots of flexibility and can be any length. You could go for literal options, or go for more abstract/symbolic (like a raindrop to symbolize feeling a bit gloomy, a keyhole for talking about struggling to open up about feelings, etc.).

...Actually, looking these up kinda makes me want to try my hand at these or at least find more examples. I've never particularly cared for poetry, but I think I genuinely enjoy these formats more than more "traditional" poems.

I’ve never written a poem in my life, the thought alone scares me, but I do journal daily for the past 6 years. The trick is to just start. I know that doesn’t sound like much of an advice, and it’s certainly not poetry-writing specific advice, but whatever plan or structure you settle on now, it will change, especially as @infpossibilityspace said, there really aren’t any rules in poetry. I maintain a poetry website and since I alone curate it, I get to read a lot of it, and it’s amazing how wild they can get, from form to content.