How to write the Graduate Coordinator regarding admission
You just write "Thank you very much for the offer.  I'm very interested, and will definitely contact you if I think of any questions.  At the moment, I'm waiting to hear back from some other institutions, but I will contact you as soon as I come to a final decision."  Graduate coordinators understand that you've applied other places, and there's a relatively low probability they are your first choice.  They've also probably sent out dozens of these offers; they're not that emotionally invested in any particular one.  Honestly, they would probably appreciate knowing where else you are considering for informational reasons.  

I hope you've informed all the places you're no longer considering that you've withdrawn your application and that you've contacted places you want to hear from to find out the situation.  There's no need to commit yourself to a timeline (what if you find out you are high on the waiting list somewhere else).