The grammar rules are simple. If you address someone by você then the imperative, as everything else, goes third person singular; if you address someone by tu, then, well, it’s second person singular:
Como está (você)? Sente-se aqui. Esteja à vontade. Tome um café, coma um bolinho.
Come estás (tu)? Senta-te aqui. Está à vontade. Toma um café, come um bolinho.
And in European Portuguese that’s that: tu is informal whereas você is semi-formal (o senhor, a senhora is formal), so you’re very conscious that tome um café, coma um bolinho implies some formality, and toma um café, come um bolinho implies familiarity; so with any particular person you either use one all the time or the other all the time, and never mix the two.
In most of Brazil tu, if used at all, and você are both informal, and so it is easier to mix the two. And it is common in colloquial language to use você together with the second person singular imperative:
Como está (você)? Toma um café, come um bolinho. Oi, você aí! Vai fora!
Now this is colloquial language, so you won't find it in conjugation tables, and you have website after website telling you that in examinations, job interviews, and other formal settings, you are required to follow proper grammar. Here’s one, and here’s another bearing on the use of the imperativo. On the other hand I would have thought Brazilians will not think you have a spoon up your nose just because you follow grammar rules for imperative use. But I am Portuguese and you may want Brazilian advice on this point.