This definitely needs to be #1 priorty in learning Korean.
In the beginning I always had gotten ㅏ(a) andㅓ(eo) mixed up with eachother along with ㅑ(ya), andㅕ(yeo), ㄱ(g/k) and ㄴ(n) for they look similar to one another.
Can you speak Korean? 한국말 할 수 있어요? hangukmal hal su isseoyo? I can speak a little Korean. 한국말 조금 할 수 있어요. hangukmal jokum hal su isseoyo.
If you use romanization it makes life and learning it so much easier, but over time you're gonna have to not use romanization in order to memorize the actual characters. Honestly this is the easiest thing to remember when learning korean and it is very crucial that you ABSOLUTELY NEED TO KNOW.