Note my changes above, marked in red. It's important to start every sentence with a capital letter and end every sentence with a single, appropriate punctuation mark.
We use "a" before words which start with a consonant sound.
We use "an" before words which start with a vowel sound.
A unicorn. (Note this one - the word is pronounced "you-nic-orn" so it starts with a consonant sound even though it starts with a vowel.)
Student or Learner