How to write the Graduate Coordinator regarding admission I have received an admission offer* from a US University for PhD program with full tuition waiver plus a TA offer. The deadline of accepting the offer  is April 15. Graduate coordinator  wrote me that 


  Please do not hesitate to contact me if you have any questions or
  concerns. We are looking forward to your response.


I have applied few other universities too and have not received their decisions. How to write the coordinator in a best way that, I'm very  much interested but I would like to notify them (acceptance or declining) regarding admission offer around the middle of March?  Will they take my view in a negative way?  

*The official certificate of admission will come from the university, after all official transcripts and other necessary documents and test scores are received; until then, this offer is considered  unofficial. You will be contacted by the university with a request for documents and for other details, but in the meantime here are the specifics of this offer.
Students who are offered a graduate assistantship are required to accept or decline by April 15, 2016. This common deadline is based on an agreement of all institutions that are  members of the Council of Graduate Schools, as described in the April 15 Resolution, a copy of which can be obtained from http://www.cgsnet.org/april-15-resolution.
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I hope you've informed all the places you're no longer considering that you've withdrawn your application and that you've contacted places you want to hear from to find out the situation.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).