Relax and enjoy what is comming towards you. Everything you will learn in aikido will take time. Dont ruine your good time by being very focussed on doing everything right from day one, because that is a very unrealistic goal. Do your best instead and remember to enjoy it.
Advice on rolling is very hard to give on a forum such as this. It is easy to give wrong advice but one also cannot control the way the advice in interpretted. Maybe some small pointers can be given to people who already mastered rolling in some degree, but for people who don't have any experience (and frankly first time rolling one week ago means no experience) should be taught how to roll by their teacher or sempai. As with all other things in aikido, one learns by doing, not by knowing.
My advice on rolling would be: go to the dojo and ask someone to help you and then practice practice and practice.
As for sitting in seiza, that is just getting used to. If it feels to painfull just go to the dojo again and have one of the sempai wacht you when you sit in seiza, just to elliminate the possibility of you sitting in a really funny way.