不 is 4th tone and is used to negate all verbs apart from one.
Examples:
不行!This is not OK
这个不好 - This is not good
我不想去 - I don't want to go
However, when 不 appears before a 4th tone word, it changes to 2nd tone.
Examples:
他不是老师 - He is not a teacher
我不去 - I'm not going
他不在 - He's not here
不要!- Don't (do that)!
这样是不对的 - Doing it this way is wrong
Phrases like 不对 and 不是 are very common. I've found that the best thing to do is to memorise the tones of these phrases, and not so much the individual characters. Obviously you want to know the tones of the characters, but when you speak you often don't have time to think of the spot - which is why learning the tones in phrases such as these is useful.
Now, there is one exception I know to this. When you say "为什么不?", meaning "why not", 不 here is 1st tone. When you 呢 to the end - 为什么不呢?, it changes back to 4th tone.
To negate 有, you use 没. 有 means "to have" or "there is/are"
我有笔 - I have a pen
她有很多朋友 - She has lots of friends
家里有人 - There are people at home
我没有笔 - I don't have a pen
她没有朋友 - She doesn't have any friends
家里没有人 - There isn't anybody at home
I hope this is helpful.
Laurence