I'm totally impressed, especially if you say you're not used to writing poetry. The rhyming is harmonious and works well. You kept the theme consistent, cauldron, conjuring/brewing a spell.

Even though it's quite good, doesn't mean it cannot be improved. Here is my feedback:

I would consider cutting "the" off before night and day. Say it out loud and see if you like it better: "At one with night, but hidden from day". Both are correct, think about it, but you can keep it as it is - it really depends your emphasis and how it sounds to you.

Secondly, I would avoid capitalising the first letter of each line, and the first letter of each rhyme. The rhyme is obvious, you do not need to force it with capitals. Let your poetry do the work, it's good.

Along with that comment, use more punctuation. "tooth of tiger, eye of snake". It would help the reader knowing where he has to pause (in a line, or between lines, punctuation helps everywhere).

Some of the lines seem to be rhyme-led (i.e. you are structuring or saying them only to fit the rhyme). I usually really challenge this and critique it, but for a beginner's effort, it's decent AND the contest requires a rhyme, so it's not totally your choice. Not a huge fan of the fourth lane (onwards the witches way). Likewise, the first two lines of the last stanza could do some rework. For instance, when spoken out loud, the word 'deadly' seems a bit different than the sentence, it sounds harsh, especially when compared to what you are saying: brewing, liquid, smoke, smell.

I hope this helped you, and I hope it has not discouraged you at all. It is a great work, especially for beginners. I will be looking forward to reading more from you :)