How should I prepare if I want to pursue architecture and study at the School of Architecture and Planning, Delhi?

Rather than preparing to join SPA Delhi, you should prepare to be an Architect or Designer. Try to learn software such as sketch up, Revit, 3D max, etc. These are fun to learn. Also, make a habit of reading articles and publishings related to Architecture and Design. Practice sketching whatever you like.

For a UG course at SPA Delhi, you will need to go through JEE Main (Paper 2).