Narita express (train)

We are heading to Tokyo for 9 nights in Aug (it’s going to be warm) but we want to climb Mount Fuji so it’s got to be with dates!

Anyway we’re landing at Narita and I’m looking at the express train we’re staying in Shinjuku, do we need to book before hand or can we get tickets when we arrive for comparable prices to booking in advance ?

Many thanks in advance