how do I calculate the firms sales, margin and net income?

Depends on what information you've been given.

Sales should not have to be "calculated." That is an account that would have a balance of what the sales were for the year.

Margin... profit margin (which is a %) or gross margin (which is the same as gross profit)?

And calculate net income? That's the number at the bottom of an income statement: revenues less expenses. This isn't something you just calculate without doing a statement.

In real life we get sales from doing journal entries, gross margin and net income by doing statements. So for a problem it has to be giving you some type of information and working backwards into these or something. We can't know what that is without that information.