The seniors are always richards to newcomers, dont worry though they just find it entertaining and it will stop after a while, best advice i can give with this is try to befriend one or more and talk to them relatively often, it helps if they are one of your friends brothers ect.
Also i dont know what you look like but dont worry about changing infront of everyone, honestly everyone could care less and dont pay any attention. Also they often make fun of the people using the cubicles to get changed so just man up and face the wall or something it will be fine.