You DO need a comma after "next" if:
* It introduces a list:
* "Next, we will discuss the budget, then the timeline, and finally the marketing strategy."
* It introduces a contrasting or opposing idea:
* "Next, I wanted to address the problem of water pollution, but we ran out of time."
* It introduces a clause that could be a complete sentence on its own:
* "Next, the team will present its findings to the board."
You DON'T need a comma after "next" if:
* It is followed by a prepositional phrase:
* "Next week, we are going on vacation."
* It is followed by a noun phrase:
* "Next step, we need to gather data."
* It is followed by a verb:
* "Next, we will start the experiment."
In short: Think about the function of "next" in the sentence. Does it introduce something new or create a pause? If so, a comma might be necessary.