Need is something that is necessary for you while ask is a mere inquiry i.e. you could do without it. They are quite different in meaning. Hope this helps, Shinji! I'd be happy to teach you something like this and more, and so help you improve in English. Please check out my profile and let me know what you think :)

Need is to require something or someone because they are essential. Ask for is to make inquiries. They are not quite the same.

‘Need’ and ‘Ask’ are completely different verbs meaning different things. So it really depends on the context. ‘Need’ implies slight desperation for example. Also, it sounds unnatural to speak in the present tense unless the context is a formal email or something similar.

1) I need your advice. This is a good sentence. 2) I ask for your advice. This doesn't make sense. Do you mean 2A) I will ask for your advice. 2B) I want to ask for your advice. 2C) I am asking for your advice.