How to say I/you should do something.

For example.

You should come to New Zealand
You should say hello
I should eat breakfast
etc

Basically could someone teach me the sentence structure for saying somebody should do something?

The word "should" in English has several different expressions in Japanese, depending on the usage.

In your first example, the usage is as a suggestion. In Japanese this is formed by using the plain (not -masu) past tense of the verb + hou ga ii desu, like this:

You should come to New Zeland
(anata *ha) nyuujiirando ni kita hou ga ii desu (yo).
(*pronounced wa)
A rough literal translation of "X verb-ta hou ga ii" is "It is better for (you) to do X".

The second example, if intended as a suggestion, would follow the same pattern as above:

You should say hello.
(anata ha) goaisatsu wo shita hou ga ii desu (yo).
(aisatsu = greeting)

If the second example is meant in the sense of "must", you would use the plain (non-masu) non-past form of the verb + beki desu. This version of "must" would indicate a sense of what is morally/situationally correct to do, as in "You should say hello, because to not do so would be wrong of you."


You should say hello.
(anata ha) goaisatsu suru beki desu.
I would warn against using this form too often because it comes off as being very strong. You might say it about the actions of a third party, but it would be better not to say it directly to the person who you think should be doing the thing. It sounds like you disaprove of the person's actions.

The third example also follows a closer usage to "must" or "have to". This one in Japanese has many different constructions. One of the textbook constructions is the negative, plain (non-masu) form of the verb minus the final i + kute *ha + ikemasen/narimasen/dame desu, etc.. (*pronounced wa). In this case, the feeling of "have to" indicates that there are consequences of not doing the thing, as in I should eat breakfast because if I don't I'll be hungry later.

I should eat breakfast.
(watashi ha) asagohan wo tabenakute ha ikemasen.
roughly, X wo verb-na kute ha ikemasen = not doing X is not good.

These last two are especially hard, but think about it this way:

I should eat breakfast.
(watashi ha) asagohan wo taberu beki desu.
I should eat breakfast (as a general rule because it's good for my health/because it was made especially for me and I would be rude not to eat it.)
vs.
asagohan wo tabenakute ha ikemasen.
I should eat breakfast (quickly or I'll be late for work/because I'll be hungry later if I don't/because I haven't eaten anything today)

Just as a note, people tend not to use any of these (especially beki) when commenting directly on the actions of people of higher status, but it can be ok to talk about someone else's/your own actions to a person of higher status in this way.

There are other words in Japanese that also mean should when the situational usage is different from your examples. Sorry that this was so long and complicated, but I hope it helps, and post again if you want to know the other ways to use the many varieties of "should" in Japanese.

Thank you for all the effort you obviously put into making that post.　Please do tell me about the other forms of should.