I am habituated to using photoshop but recently krita got me interested in using the software with its cool features that too for free. I want to know how can one simulate pressure when drawing without the use of a drawing tablet. Could anyone please tell.

Properly speak, you cannot, of course. The only things that we can get from a mouse are speed and distance, and you can bind those inputs to any expression you want in the brush editor, like size or opacity.