Remember, don't use Romanization! I know you're just learning and might not have the proper keyboard yet, and you might learn that way at school, but don't don't don't! :) I think this is good evidence why. Such a disconnect between the highly irregular Latin alphabet (and English pronunciation) and 한글.
But anyways, don't worry about messing up. Like you said, that's how you learn (as long as you get feedback and corrections). Just keep asking questions!
Oh yeah, anyways I see now that you said you never wrote them down in 한글 :( That's silly...and confusing for you!
1. 안녕하세요 - informal, honorific (standard way of saying "Hello" or "How are you?")
안녕하십니까 - formal, honorific (a lot less common and even less useful for you at this point. It's super formal)
NOTE: When ㅂ comes before ㄴ, it is pronounced as ㅁ(I noticed you wrote in your guess '므'). Also, 까 is the formal question ending. So little hint for spelling (you wrote '가'). Remember, 안녕하세요 and 안녕하십니까, while translated as "Hello", are actually questions that literally mean 'Are you at peace?'
2. 처음 뵙겠습니다
NOTE: Same thing, ㅂ before ㄴ = ㅁ SOUND but not spelling. This is a statement, so it wouldn't end in 까. Yep, "I'm seeing you for the first time"
5. 만나서 반갑습니다
NOTE: Same thing, ㅂ before ㄴ sounds like ㅁ. I see and hear and use this one a lot more often than #2. Literally it's more like "I'm pleased to meet you"
잘 하셨어요~! (You did well...aka good job!)
You can use talktomeinkorean.com to reinforce these too.