Hii Abul,
The admission for ba honours in Sikkim will start after the declaration of board results that is may so the forms will be out after a month of exam so you can fill the form when the forms will be out(April).In the college's of your interest.
Some of the good colleges are
. Sikkim University
.Nar Bahadur Bhandari degree college
. Loyola College of education
. Sikkim Manipal University
These are few of the colleges in Sikkim rest you can apply according to your choice.