can we use consont "a" for uniform?

As a general rule, "a" is used when the first sound (not letter) in a word is a consonant. "An" is used when the first sound (not letter) is a vowel. "Uniform" is pronounced /ˈjuːnɪfɔːm/ or /ˈjunəfɔrm/, the first sound being /j/, (sounds like the 'y' in the words "you" or "yes"), which is a consonant.