The imperative form, known as the Befehlsform in German, is used to modify the verb when you are giving a command (including polite commands!). Sentences using the imperative are often followed by an exclamation mark to reinforce their meaning.
Forming the imperative is simple. There are rules for du, ihr, and Sie conjugations of the verb. When using the du conjuncation, simply take the -st ending away, remove any umlauts, and you have the imperative form. For example, 'Go home!' could be translated as 'Geh nach Hause!' or 'Fahr nach Hause!' (note the removal of the umlaut from du fährst).
The ihr and Sie conjungations are even simpler. For ihr, you simply remove the ihr and use the verb on its own – for example, 'Fahrt nach Hause'. For Sie, simply conjuncate the verb as normal, but ensure that it comes first in the sentence – for example, 'Gehen Sie nach Hause!'. Here the exclamation mark helps to convey meaning, showing that the sentence is a command rather than a question.
Irregular verbs can be exceptions to these rules: in particular, the verb Sein uses different rules, which you must simply learn. The du form is sei, the ihr form is seid, and the Sie form is Seien Sie. For example, 'Sei ruhig!', 'Seid ruhig!' and 'Seien Sie (bitte) ruhig!'