Unlike English, vowel sounds in Spanish never change, which makes it wonderfully easy for us learners!
a – always like the “a” in “apple”
e – always like the “e” in “elephant”
i – always like the “i” in “ski”
o – always like the “o” in “orange”
u – always like the “oo” in “oops”
y – sometimes a vowel, and pronounced in the same way as the “i” Spanish vowel
Practise saying them out loud. If your “o” in “hola” sounds like the “o” in “home”, change it and practise over and over until you say it correctly. Get someone else to listen to you and tell you if you’ve got it.