Translation:Please help me, I am a beginner.
I think "newbie" should be accepted as an alternative to "beginner". This is a slang lesson after all.
Good suggestion. I take it that 菜鸟 wouldn't really be used in a conversation that's more on the formal side?
It's pretty normal to use it in a conversation. Also for professional athlete's rookie year, we will call it 菜鸟年. However, you probably won't use it in a formal document.
According to an online dictionary, 菜 is also slang for "bad" or "below average" (as in, not skilled at something, etc.)
Pleco translates it as a colloquial term for "poultry raised for meat" as well as "rookie" so maybe it has the same sense as English's freshman/fresh-meat wordplay? As a beginner in a game you're going to get metaphorically slaughtered?
I have an impression of vegetable in the word 菜. A weaker bird who only eats vegetables. He is not strong enough to catch meat or does not have a technique to do so. He is not an eagle, or a hawk to fight against others.
I am Japanese, so the above is just a guess.
It's from Taiwanese, and it's informal. It means someone is the newcomer in a team, and the senior staff is called "老鳥（old bird)". Although the newcomer is usually bad in skills, this word actually often emphasizes the social class. (The newbie should listen to the senior.)
Is 菜鸟 only net slang, or can I use the term in everyday speech as well (without looking like an idiot)?
We don't really use this word per se now, but you can say "我很菜" or "我很辣鸡（a less agressive way to say"垃圾"）" online or irl to express that you are not good at whatever you are doing.