You can only get admission in MCA in NITs through NIMCET as there is no management or direct seat quota so I would suggest you to prepare for the entrance test and get decent score so that you can easily get admission there in top NITs. Also, as of now the dates are not yet announced but as per previous year information the forms should be released in the month of Feb 2018 and exam should  be held in the month of May. For more updates, stay tuned on below provided website -

https://it.careers360.com/

Good Luck!!!!