Many people, who have studied English for years, have difficulties with casual conversation. One reason for this problem is that English speakers often add an extra word onto the front of a sentence. Native speakers know to just ignore such words. These words include several grammatical types, including particles, connectives, expletives, prepositional phrases and adverbs. They have no actual dictionary meaning and are called discourse markers. Their function is to show that you are ready to speak or want to keep speaking. Also using a discourse marker gives the speaker a pause while they think of what to say.
Normally the word no is a negative adverb or a determiner. Sometimes words like this one are used at the beginning of a sentence as a discourse marker and it means nothing. For example, "No, I agree with you." and "Yes, oh no, I agree with that, but when you're cutting the meat, what are your butchers doing?" . In this use the discourse marker can cause confusion because it disagrees with the rest of the sentence.
These words are commonly used as discourse markers:
for a start
as I say
to begin with
as you said
OK / okay
on the one hand
on the other hand
(Various words unfit to print in a family publication are also used in this manner. These have, of course, been excluded from the list above.)
Native speakers avoid using discourse markers in formal English and should do the same when talking to second-language speakers. However this use is a strongly ingrained habit that is difficult to break. So when I say "no" at the beginning of a sentence, and then agree with you, forgive me.
It is impractical to memorize a list which is used so loosely. If you can ignore the first word in a sentence, especially in spoken English, it is often better to ignore it. It especially should be ignored if the meaning is clearer without that first word. English listening and speaking practice needs to be done outside of the limitations of the classroom. Getting used to discourse markers is not possible without such practice.