Train ticket booking in advance

We, husband and wife and 7 years old son, are arriving Shanghai on 18/9/2014 by China Eastern Airlines, will take train from Shanghai to Chengdu by Z122 on 21/9/2014, and from Chengdu to Xian by train K6 on 28/9/2014, and then from Xian to Yinchuan by train K1085 on 03/09/2014 and three bookings we prefer Soft Sleeper (lower).We will appreciate if you please advise us (I) when should we book (ii) can we use your services to book well in advance to ensure confirm booking and (iii) how do we make payments. Regards