Hi Rudra! 
As you know, B. Arch is a bachelor degree course in architecture. Those who wants to study architecture after their 12th, they can pursue B. Arch course. But there are all India entrance exams to get B. Arch in your preferred university. Different entrance exams are accepted by different colleges. Here are top 5 colleges for B. Arch in India for you:

• School of planning and architecture
• National Institute of technology
• Indian Institute of technology Kharagpur and Roorkee
• Jadhavpur University
• Maulana Abul kalam azad university of technology

There are mainly 2 exams for B. Arch, there are other entrance exams for other colleges too. But these 2 exams are massively accepted by top notch colleges. So majority preferred these two exams. Such as: NATA and IIT JEE mains paper 2. To get admission in top notch colleges, candidates have to apply for one of these exam on the basis of what their preferred college will accept. Candidates have to qualify for the exam and have to get above cut off marks. 

What subject to study to get B. Arch

• NATA-

National Aptitude Test In Architecture is a national level entrance exam conducted by council of architecture for admission to 5 years B. arch degree course at various institutions in India. There is no age eligibility criteria for NATA mentioned by CoA in particular.

Syllabus- 

• Drawing includes memory drawing, shades, shadows, 2D compositions, visualisation, perspective drawing etc. NATA drawing part is totally depend on your creativity and ability. You need to practice more and more to improve your skills and ideas. You need to be creative, because creativity is necessary in this field. 
• Aptitude test includes Diagrammatic Reasoning, Numerical Reasoning, Verbal Reasoning, Inductive Reasoning, Situational Judgment, Logical Reasoning, Abstract Reasoning. 
• Mathematics include algebra, logarithm, matrices, trigonometry, coordinate geometry, 3D coordination of geometry, theory of calculus, application of calculus, permutation and combination, statistics and probability.
• NATA also includes questions from Physics and chemistry in which class 10-12 studies are important. As well as includes general aptitude test with mathematical reasoning, sets and relations.

• JEE- MAIN paper2-

Joint Entrance Exam – Main is an national level all India exam conducted by  NTA to take admission of the candidates who are willing to pursue architecture must go for IIT JEE mains paper 2. There are also no age limit to sit for JEE- MAIN.

Syllabus- 

• Mathematics and Aptitude part will have questions like objective type and numerical value. This part will have questions on such topics- 

Awareness of persons, places, Buildings, Materials. Objects, Texture related to Architecture and build-environment. Visualising three-dimensional objects from two-dimensional drawings. Visualising. different sides of three-dimensional objects. Analytical Reasoning Mental Ability (Visual, Numerical and Verbal)

• Drawing part will have questions on drawing and sketching to evaluate candidate's creative skills. This will consist questions on such topics-

Drawing and Designing of geometrical or abstract shapes & patterns in pencil, Transformation of forms both 2D and 3D union, Elevation and 3D views of objects rotation, Development of Surfaces and Volumes, Generation of Plan, Creating 2D & 3D compositions using given shapes and forms, Sketching of activities from memory of urban scenes.

Hope these information will help you!