まず + [sentence] まずは + [sentence]
まず means "first", "to start with": this step comes before the others. It is an adverb and usually stands at the front of the sentence or clause.
In a list of steps, まず opens the list and つぎ次に, それから and さいご最後に carry it on.
まず also puts one thing ahead of everything else, without a second step in view. まずは, with は, is the same and very common in speech.
まず orders steps. "The first time" something ever happened is はじめて初めて, or さいしょ最初に for the first of several occasions.
With a guess (きごう〜だろう, きごう〜とおもう思う) or a negative, まず means "in all likelihood". Here it says how sure the speaker is, not which step comes first.