に marks a destination or something toward which an action/movement is directed. Typical verbs that are used with particle に with this meaning are:
のる(get on; ride), とまる(stay overnight), のぼる(climb), はいる(enter), つく(arrive), すむ(live)
わたしは しんじゅく駅で 電車に のります。
I get on the train at Shinjuku Station.
おとうとは 来年 しょうがっこうに 入ります。
My younger brother will enter elementary school next year.