Niemand is synonymous with keiner ‎(“no one”), which is the nominalised masculine form of the pronoun kein ‎(“no”). In written German, niemand is more common than keiner, whereas colloquial German prefers the latter. (Note that keiner can also be used in other senses, in which niemand is not applicable.)
Niemand may or may not take the case endings -em (dative) and -en (accusative). Thus: “Ich suche niemand” and “Ich suche niemanden” are both correct translations of “I'm looking for nobody.” The latter is probably somewhat more common (at least in writing).
With a following adjective, niemand is always uninflected. The adjective itself is capitalized and declined in the strong pattern. It generally uses neuter forms: niemand Neues – "nobody new". In the accusative case it may also take a masculine form: “Ich suche niemand Neuen” alongside “Ich suche niemand Neues.” ("I'm looking for nobody new.") The dative form has -em either way. The genitive case cannot be used with an adjective, but needs to be paraphrased.