Sorry to know that you have not been paid salary.
You have option of sending legal notice to them for payment of salary-however you do not seem to have documentary poof of employment-never the less do send them a legal notice.
There is no standard letter of resignation nor is there any standard format of appointment letter.
You may send your resignation letter by speed post stating that you were appointed as----- (designation ) on ---(date and year) and are resigning as you have not been paid salary.
Also say that no notice period is applicable as you were on probation as informed during interview.
15th October 2016 From India, Delhi