"A" vs "An" : I use a uniform and I will need an umbrella

Use of indefinite articles “a” & “an” 

The article “a” is used when the following word begins with a consonant sound.

El artículo “a” (un, uno, una) se usa cuando la palabra que sigue comienza con un sonido consonante.

-       I am eating a red apple
-       John applied to a university /juːnɪˈvəːsɪti/ (the ‘u’ in university is a consonant sound ‘ju:’)
-       It is a hot day

The article “an” is used when the following word begins with a vowel sound.

El artículo “an” (un, uno, una) se usa cuando la palabra que sigue comienza con un sonido vocal.

-       I am eating an apple
-       I need an umbrella /ʌmˈbrɛlə/  

-       He waited for an hour /ˈaʊə/ (the ‘h’ in hour is silent, so it starts with a vowel sound)

More about “a” vs “an”: 

• Merriam Webster's Video
• Everything English Blog