hello, 

here is a link to know all about JOSAA

https://engineering.careers360.com/articles/josaa-counselling

and for payment

Candidates who have been allotted through the counselling of JoSAA 2018 are required to download their “Provisional Allotment Letter”. The e-challan for payment of seat acceptance fee also has to be downloaded by the candidates. Payment of Rs. 35,000 by the general candidates and Rs. 15,000 by the SC/ST/Gen-PwD/OBC-NCL-PwD/SC-PwD or ST-PwD candidates have to be done through e-Challan or SBI Net Banking.