问一个offer letter的问题

请问下面的offer  应该怎么应对？  能不能先接受 然后 4月15之前拒掉？  这个也没说回复时间啊 求指教
学校来的email 是这么说的：
Greetings and Congratulations!

I am writing to inform you that your graduate admission application was reviewed by the School of Computing and Information Sciences graduate admissions committee and was recommended for admission for the Fall Term 2013.

I am also very pleased to inform you that we would like to offer you a full Graduate Assistantship.  The terms and conditions will be outlined in the offer letter that we would like to forward to you via Express Mail.

We hope you will accept our offer and we look forward to meeting you in the Fall semester.  I ask that you kindly reply as soon as possible with your current mailing address including postal code and telephone number.  I will email the official offer letter (pdf format), and the original will be forwarded to you along with the I-20 at a later date.