My cellular data is light gray...won't let me turn on ???

My cellular data is light gray..won't let me turn on to create an account. I did just buy an iPhone today and had to change my apple I'd/password because the one I use everyday on this iPad wouldnt recognize. So now everything is under new apple I'd / password. I never created my cellular acct because I didn't need it until now..... Now I can't get it to work. Please help. Thx