DO i need a passport? 

Please reference the State Department and this site for more information and pay attention to disclaimers: https://www.celebritycruises.com/blog/do-you-need-a-passport-for-a-cruise
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I want a cabin for one. can i have a cabin for one?

You can have a cabin for one, but you're going to have to pay for a cabin for two.  All prices are based on double occupancy in each cabin. 

I don't know who i am going to travel with yet. should i wait?

That's fine.  Just book the cabin know you will have to pay for two if you don't find that roomie. You can add a name up until right before sailing.