Hello,

You can join for BArch through jee mains paper2 entrance exam score. The entrance exam has 3 sections which has questions from :-

1. Mathematics
2. general aptitude
3. drawing skills.

You apply for form next year which may be released december of current year. Meanwhile you can start preparing for the entrance exam. Here is the link for JEE mains Paper2 syllabus :-

https://engineering.careers360.com/articles/jee-main-paper-2-syllabus

The above article has given percentage value of importance of subjects. you can plan  how to prepare accordingly. Here is the link of previous year paper analysis:-

https://engineering.careers360.com/articles/jee-main-paper-2-analysis

it will help you know how question are prepared and how they vary each year.

Hope it helps!