Since Nicodemus has little concept of understanding renewal and Spirit (which are concepts he should understand), Jesus would begin to explain more information that is useful for all of us.
First up is how at that time, the Son of Man is the only being (alive) to go up to heaven (3:13). Now, there are some questions that arise from it because of people like Elijah who also seemed to arrive to heaven. However, all those people back then (aside from Jesus) never came back down to Earth, and Jesus was likely referring to people still alive. Even then, note that the Greek for "ascend" (Strong 305) has the impression of climbing or going up, and only the Son of Man gets into heaven by His own ability. All the other people require God to carry them into heaven. Thus, Jesus has some unique features that further indicate that he is a very important person.
Next, Jesus compared lifting up the Son of Man (AKA the resurrection) to Moses lifting the serpent (3:14). Back in Numbers 21:8-9, God told Moses to make a bronze snake on a pole so the people cursed by a snake (from self-inflicted side effects) can look to it and be cured.
In a similar way, when Jesus is lifted up, anyone (including YOU!) who "believes in Him" can have eternal life (3:15). In fact, since the serpent event involves recovery, believing in Jesus is how the renewal and "born again" happens because broken things need renewing (and if we're honest, we have some issues in ourselves, which is why Jesus talked about the need to be "born again").